COMPAT: pandas private API

[jreback]

So making some changes here that could potentially break some of the de-facto private API: pandas-dev/pandas#13147

IOW if doing:

from pandas.core import common as pdcom

pdcom.is_categorical()

for example. This wont' break now as I left it in the namespace, but we are migrating to
``pandas.api` for these types of things. However we won't be able to issue a deprecation warning probably to the next version.

[gfyoung]

Update: pydata/pandas#13147 has been merged, though in the interests of backwards compatibility, I don't imagine much can be done on this end unless we want to add some not-so-nice try-except blocks.

[bashtage]

This doesn't appear to be an issue.