ENH: serialization of categorical to HDF5 (GH7621)
BLD: fix odd usage of cython imports, xref (GH8602)
Merge pull request #8793 from jreback/cat_hdf

ENH: serialization of categorical to HDF5 (GH7621)
DOC: v0.15.2 edits
DOC: v0.15.2 typos
Add test for unicode separator and data
ENH: Allow unicode separators in to_csv & read_csv

* unicode delimiters coercible to ASCII (e.g., `u(',')`) are always allowed.
* unicode delimiters that don't fit within ASCII are always disallowed
  with C engine.
* unicode delimiters that don't fit within ASCII will never work in
  Python 2 engines (because their CSV assumes seps are unicode).

Closes #6035.
Fix check for text type to use same fallback stuff [and only check for ord when it's not bytes
PEP8 cleanup of my checkin