document the changes

Author: mattip

doc/source/whatsnew/v0.21.0.txt

@@ -332,6 +332,9 @@ Performance Improvements
 Bug Fixes
 ~~~~~~~~~
 
+- Compatibility with PyPy increased by removing the assumption that sets are sorted in ``usecols``
+  (of ints) and by checking for a list object before calling PyList_GET_SIZE, which in both
+  cases may be considered bugs 
 
 Conversion
 ^^^^^^^^^^
@@ -358,6 +361,8 @@ Indexing
 - Allow unicode empty strings as placeholders in multilevel columns in Python 2 (:issue:`17099`)
 - Bug in ``.iloc`` when used with inplace addition or assignment and an int indexer on a ``MultiIndex`` causing the wrong indexes to be read from and written to (:issue:`17148`)
 - Bug in ``.isin()`` in which checking membership in empty ``Series`` objects raised an error (:issue:`16991`)
+- Tests split into cases for CPython and PyPy where needed, which highlights the fragility
+  of index matching with ``float('nan')``, ``np.nan`` and ``NAT``.
 
 I/O
 ^^^