Merge pull request #4990 from jreback/cat_fix

CLN: fix py2to3 issues in categorical.py
TST: fix indexing test for windows failure
Merge pull request #4992 from Komnomnomnom/indexing-dtype-test-fix

TST: fix indexing test for windows failure
BUG: Fix appending when dtypes are not the same (error showing mixing float/np.datetime64) (GH4993)