CLN: remove PY2
master issue for removing all vestigaes of Python 2.

After merging: #24942 (removing the PY2 CI jobs) and #25554 (bump numpy to 1.13.3), we will be ready to remove PY2 code, tests, and doc references.

Let's do this in bite sized pieces. IOW don't do a PR with changes for everything.

cc @pandas-dev/pandas-core 