Follow-up to #17419
This moves a few other 3ish line functions from tslib to tslibs.timezones.

One non-trivial function `_get_zone` is cut/pasted over.

Replaces imports in inference.pyx and period.pyx with imports from tslibs.timezones.  In the inference, case, replaces the import of the python version `get_timezone` with the cython version `_get_zone`

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
