MacPython linux build failing 
There are build failures with the manylinux1 32-bit docker image. 

```
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -DNPY_NO_DEPRECATED_API=0 -Ipandas/_libs/tslibs -I./pandas/_libs/tslibs -I/opt/python/cp38-cp38/lib/python3.8/site-packages/numpy/core/include -I/opt/python/cp38-cp38/include/python3.8 -c pandas/_libs/tslibs/period.c -o build/temp.linux-x86_64-3.8/pandas/_libs/tslibs/period.o
pandas/_libs/tslibs/period.c: In function ‘__pyx_f_6pandas_5_libs_6tslibs_6period_pqyear’:
pandas/_libs/tslibs/period.c:12464:3: warning: ‘__pyx_v_year’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   return __pyx_r;
   ^
pandas/_libs/tslibs/period.c: In function ‘__pyx_f_6pandas_5_libs_6tslibs_6period_pquarter’:
pandas/_libs/tslibs/period.c:12512:3: warning: ‘__pyx_v_quarter’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   return __pyx_r;


building 'pandas._libs.hashtable' extension
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wl,-strip-all -I/usr/local/include -fPIC -DNPY_NO_DEPRECATED_API=0 -Ipandas/_libs/src/klib -I/opt/python/cp38-cp38/lib/python3.8/site-packages/numpy/core/include -I/opt/python/cp38-cp38/include/python3.8 -c pandas/_libs/hashtable.c -o build/temp.linux-i686-3.8/pandas/_libs/hashtable.o -Werror
pandas/_libs/hashtable.c: In function ‘__pyx_f_6pandas_5_libs_9hashtable_value_count_float64’:
pandas/_libs/hashtable.c:27378:41: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
pandas/_libs/hashtable.c:29050:41: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (__pyx_t_10 = 0; __pyx_t_10 < __pyx_t_9; __pyx_t_10+=1) {
                                         ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_8duplicated_uint64’:
pandas/_libs/hashtable.c:29800:38: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
             __pyx_t_10 = ((__pyx_v_k != __pyx_v_table->n_buckets) != 0);
                                      ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_10ismember_uint64’:
pandas/_libs/hashtable.c:30271:177: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
           *__Pyx_BufPtrStrided1d(__pyx_t_5numpy_uint8_t *, __pyx_pybuffernd_result.rcbuffer->pybuffer.buf, __pyx_t_17, __pyx_pybuffernd_result.diminfo[0].strides) = (__pyx_v_k != __pyx_v_table->n_buckets);
                                                                                                                                                                                 ^
pandas/_libs/hashtable.c: In function ‘__pyx_f_6pandas_5_libs_9hashtable_value_count_object’:
pandas/_libs/hashtable.c:30731:33: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
   for (__pyx_t_8 = 0; __pyx_t_8 < __pyx_t_7; __pyx_t_8+=1) {
                                 ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_14duplicated_object’:
pandas/_libs/hashtable.c:31389:31: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
       __pyx_t_9 = ((__pyx_v_k != __pyx_v_table->n_buckets) != 0);
                               ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_16ismember_object’:
pandas/_libs/hashtable.c:31825:171: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
     *__Pyx_BufPtrStrided1d(__pyx_t_5numpy_uint8_t *, __pyx_pybuffernd_result.rcbuffer->pybuffer.buf, __pyx_t_16, __pyx_pybuffernd_result.diminfo[0].strides) = (__pyx_v_k != __pyx_v_table->n_buckets);
                                                                                                                                                                           ^
pandas/_libs/hashtable.c: In function ‘__pyx_f_6pandas_5_libs_9hashtable_value_count_int64’:
pandas/_libs/hashtable.c:32258:39: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (__pyx_t_9 = 0; __pyx_t_9 < __pyx_t_8; __pyx_t_9+=1) {
                                       ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_20duplicated_int64’:
pandas/_libs/hashtable.c:33007:38: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
             __pyx_t_10 = ((__pyx_v_k != __pyx_v_table->n_buckets) != 0);
                                      ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_22ismember_int64’:
pandas/_libs/hashtable.c:33478:177: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
           *__Pyx_BufPtrStrided1d(__pyx_t_5numpy_uint8_t *, __pyx_pybuffernd_result.rcbuffer->pybuffer.buf, __pyx_t_17, __pyx_pybuffernd_result.diminfo[0].strides) = (__pyx_v_k != __pyx_v_table->n_buckets);
                                                                                                                                                                                 ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_24mode_float64’:
pandas/_libs/hashtable.c:33800:41: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (__pyx_t_13 = 0; __pyx_t_13 < __pyx_t_12; __pyx_t_13+=1) {
                                         ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_26mode_int64’:
pandas/_libs/hashtable.c:34225:41: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (__pyx_t_13 = 0; __pyx_t_13 < __pyx_t_12; __pyx_t_13+=1) {
                                         ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_28mode_uint64’:
pandas/_libs/hashtable.c:34650:41: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (__pyx_t_13 = 0; __pyx_t_13 < __pyx_t_12; __pyx_t_13+=1) {
                                         ^
pandas/_libs/hashtable.c: In function ‘__pyx_pf_6pandas_5_libs_9hashtable_30mode_object’:
pandas/_libs/hashtable.c:35077:35: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
   for (__pyx_t_13 = 0; __pyx_t_13 < __pyx_t_12; __pyx_t_13+=1) {
                                   ^
cc1: all warnings being treated as errors

```

To reproduce

```Dockerfile
FROM quay.io/pypa/manylinux1_i686:latest

RUN git clone --depth=1 https://github.com/pandas-dev/pandas \
 && cd pandas \
 && /opt/python/cp38-cp38/bin/python -m pip install numpy cython python-dateutil pytz
WORKDIR pandas

RUN /opt/python/cp38-cp38/bin/python setup.py build_ext -i
```

I've uploaded the generated hashtable.c file [here][hashtable].


[hashtable]: https://gist.github.com/1f60b2059a103e6d36c02a0d07200e59


---

#34409 fixed the following issues.

https://dev.azure.com/pandas-dev/pandas-wheels/_build/results?buildId=35250&view=logs&j=e9afaa34-1a0f-534e-78fc-fae528ccd915&t=4685e914-c192-594f-c25f-e6cef98b7ba4

```
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wl,-strip-all -I/usr/local/include -fPIC -DNPY_NO_DEPRECATED_API=0 -Ipandas/_libs/tslibs -I./pandas/_libs/tslibs -I/opt/python/cp38-cp38/lib/python3.8/site-packages/numpy/core/include -I/opt/python/cp38-cp38/include/python3.8 -c pandas/_libs/tslibs/period.c -o build/temp.linux-x86_64-3.8/pandas/_libs/tslibs/period.o -Werror
pandas/_libs/tslibs/period.c: In function ‘__pyx_f_6pandas_5_libs_6tslibs_6period_pqyear’:
pandas/_libs/tslibs/period.c:12479:3: error: ‘__pyx_v_year’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   return __pyx_r;
   ^
pandas/_libs/tslibs/period.c: In function ‘__pyx_f_6pandas_5_libs_6tslibs_6period_pquarter’:
pandas/_libs/tslibs/period.c:12527:3: error: ‘__pyx_v_quarter’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   return __pyx_r;
   ^
cc1: all warnings being treated as errors
warning: pandas/_libs/groupby.pyx:1101:26: Unreachable code
```

https://dev.azure.com/pandas-dev/pandas-wheels/_build/results?buildId=34768&view=results from May 3rd may be the first one with this warning. https://dev.azure.com/pandas-dev/pandas-wheels/_build/results?buildId=34727&view=results from May 2nd built successfully.