BUG: NUMPY_IMPORT_ARRAY_RETVAL undeclared when installing pandas 0.18.1
I've suddenly started getting the following error when installing pandas 0.18.1 via pip on Python3.6:

```
building 'pandas.json' extension
  creating build/temp.linux-x86_64-3.6/pandas/src/ujson
  creating build/temp.linux-x86_64-3.6/pandas/src/ujson/python
  creating build/temp.linux-x86_64-3.6/pandas/src/ujson/lib
  x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fPIC -Ipandas/src/ujson/python -Ipandas/src/ujson/lib -Ipandas/src/datetime -Ipandas/src/klib -Ipandas/src -I/tmp/pip-install-8vres3tf/pandas/.eggs/numpy-1.19.0-py3.6-linux-x86_64.egg/numpy/core/include -I/usr/include/python3.6m -I/var/lib/go-agent/pipelines/make-ci-01/code/.venv/include/python3.6m -c pandas/src/ujson/python/ujson.c -o build/temp.linux-x86_64-3.6/pandas/src/ujson/python/ujson.o -D_GNU_SOURCE -Wno-unused-function
  x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fPIC -Ipandas/src/ujson/python -Ipandas/src/ujson/lib -Ipandas/src/datetime -Ipandas/src/klib -Ipandas/src -I/tmp/pip-install-8vres3tf/pandas/.eggs/numpy-1.19.0-py3.6-linux-x86_64.egg/numpy/core/include -I/usr/include/python3.6m -I/var/lib/go-agent/pipelines/make-ci-01/code/.venv/include/python3.6m -c pandas/src/ujson/python/objToJSON.c -o build/temp.linux-x86_64-3.6/pandas/src/ujson/python/objToJSON.o -D_GNU_SOURCE -Wno-unused-function
  In file included from /tmp/pip-install-8vres3tf/pandas/.eggs/numpy-1.19.0-py3.6-linux-x86_64.egg/numpy/core/include/numpy/ndarraytypes.h:1822:0,
                   from /tmp/pip-install-8vres3tf/pandas/.eggs/numpy-1.19.0-py3.6-linux-x86_64.egg/numpy/core/include/numpy/ndarrayobject.h:12,
                   from /tmp/pip-install-8vres3tf/pandas/.eggs/numpy-1.19.0-py3.6-linux-x86_64.egg/numpy/core/include/numpy/arrayobject.h:4,
                   from pandas/src/ujson/python/objToJSON.c:40:
  /tmp/pip-install-8vres3tf/pandas/.eggs/numpy-1.19.0-py3.6-linux-x86_64.egg/numpy/core/include/numpy/npy_1_7_deprecated_api.h:17:2: warning: #warning "Using deprecated NumPy API, disable it with " "#define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
   #warning "Using deprecated NumPy API, disable it with " \
    ^
  pandas/src/ujson/python/objToJSON.c: In function ‘initObjToJSON’:
  pandas/src/ujson/python/objToJSON.c:201:10: error: ‘NUMPY_IMPORT_ARRAY_RETVAL’ undeclared (first use in this function)
     return NUMPY_IMPORT_ARRAY_RETVAL;
            ^
  pandas/src/ujson/python/objToJSON.c:201:10: note: each undeclared identifier is reported only once for each function it appears in
  pandas/src/ujson/python/objToJSON.c:202:1: warning: control reaches end of non-void function [-Wreturn-type]
   }
   ^
  error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
  ----------------------------------------
  ERROR: Failed building wheel for pandas
```

This has been working fine for quite some time already and I suspect this is related to the recent release of numpy-1.19.0.

I've seen there was a similar issue here: https://github.com/pandas-dev/pandas/issues/5326, but there was no mention of undeclared variable`NUMPY_IMPORT_ARRAY_RETVAL` in there.

What's strange to me is the fact that pandas is attempting to build numpy-1.19.0, whereas I have numpy-1.17.4 in requirements. Is it using most recent numpy to build the ujson extension and doesn't look at requirements for that purpose at all?