Clean Up C Warnings
Getting closer to no warnings (at least with clang). Changes in this PR get rid of the following warnings:

```sh
warning: pandas/_libs/window/aggregations.pyx:60:4: Buffer unpacking not optimized away.
warning: pandas/_libs/window/aggregations.pyx:60:4: Buffer unpacking not optimized away.
warning: pandas/_libs/window/aggregations.pyx:60:36: Buffer unpacking not optimized away.
warning: pandas/_libs/window/aggregations.pyx:60:36: Buffer unpacking not optimized away.

pandas/_libs/hashing.c:25100:20: warning: unused function '__pyx_memview_get_object' [-Wunused-function]
  static PyObject *__pyx_memview_get_object(const char *itemp) {
                   ^
pandas/_libs/hashing.c:25105:12: warning: unused function '__pyx_memview_set_object' [-Wunused-function]
static int __pyx_memview_set_object(const char *itemp, PyObject *obj) {

pandas/_libs/algos.c:81410:3: warning: code will never be executed [-Wunreachable-code]
  __Pyx_SafeReleaseBuffer(&__pyx_pybuffernd_arr.rcbuffer->pybuffer);
  ^~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/algos.c:83045:3: warning: code will never be executed [-Wunreachable-code]
  __Pyx_SafeReleaseBuffer(&__pyx_pybuffernd_arr.rcbuffer->pybuffer);
  ^~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/algos.c:83951:3: warning: code will never be executed [-Wunreachable-code]
  __Pyx_SafeReleaseBuffer(&__pyx_pybuffernd_arr.rcbuffer->pybuffer);
  ^~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/algos.c:84857:3: warning: code will never be executed [-Wunreachable-code]
  __Pyx_SafeReleaseBuffer(&__pyx_pybuffernd_arr.rcbuffer->pybuffer);
  ^~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/algos.c:85034:3: warning: code will never be executed [-Wunreachable-code]
  __Pyx_SafeReleaseBuffer(&__pyx_pybuffernd_arr.rcbuffer->pybuffer);
  ^~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/algos.c:85940:3: warning: code will never be executed [-Wunreachable-code]
  __Pyx_SafeReleaseBuffer(&__pyx_pybuffernd_arr.rcbuffer->pybuffer);
  ^~~~~~~~~~~~~~~~~~~~~~~

pandas/_libs/tslibs/conversion.c:33113:20: warning: unused function '__pyx_memview_get_object' [-Wunused-function]
  static PyObject *__pyx_memview_get_object(const char *itemp) {
                   ^
pandas/_libs/tslibs/conversion.c:33118:12: warning: unused function '__pyx_memview_set_object' [-Wunused-function]
static int __pyx_memview_set_object(const char *itemp, PyObject *obj) {
```

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