Pypy fixes
benign changes visa-vis CPython needed to build pandas for PyPy 2.7
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15080?src=pr) is 84.77% (diff: 100%)
> Merging [#15080](https://codecov.io/gh/pandas-dev/pandas/pull/15080?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15080   diff @@
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  Files           145        145          
  Lines         51212      51220     +8   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43413      43421     +8   
  Misses         7799       7799          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [42cdb34...b38d7be](https://codecov.io/gh/pandas-dev/pandas/compare/42cdb348cd3800157e04d27b13a7dd0bf52a0d2b...b38d7bee3c7fe25972dc9d3e9620fc95ac0e615b?src=pr) commit 2e3c256 fixes a bug where substitution of ``PyString_GET_SIZE`` with ``PyUnicode_GET_SIZE`` always happened, on both python 2 and 3. It just turns out that the c-macros ``PyUnicode_GET_SIZE`` and ``PyString_GET_SIZE`` are equivalent on CPython, so the error compiled and ran as advertised. The issue manifested itself on PyPy which is more particular. [EDIT upon review and updated pull request this became changeset 91522c6, thanks for the corrections] [EDIT the suggested fix didn't work, revert to working version} updated pull request as per reviews and suggestions An alternative to the ``PyString_GET_SIZE`` vs. ``PyUnicode_GET_SIZE`` would be to just use ``Py_SIZE`` instead on obth python2 and python3, which is less clear but works. Any opinions? @mattip FYI I have https://github.com/pandas-dev/pandas/pull/15101 to more generally fix the dealloc issue. @jreback cool, #15101 makes my changes redundant. So in light of the unsatisfactory solution to the ``PyString_GET_SIZE`` vs ``PyUInicode_GET_SIZE`` problem, and #15101, I will close this pull request, and resubmit only the change to pandas/src/ujson/python/JSONtoObj.c  Sorry for the noise