COMPAT: pandas private API

So making some changes here that could potentially break some of the de-facto private API: pydata/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.

Start by reading the linked pandas issue, pydata/pandas#13147, then search the Blaze codebase for imports or uses of pandas.core.common and the referenced pdcom.is_categorical API. Determine which private APIs are affected by the migration to pandas.api; the issue is complete when the compatibility impact and required changes are established.

Written by the indexing model from the issue text.