REF: stricter typing, better naming in parsing.pyx
Trying to move towards having fewer different string parsing paths.
Thanks @jbrockmendel  Because I couldn't find it in the changelog (probably because it's supposed to be an internal function no one should have actually used):
This PR removed the `parse_time_string` function from `pandas.core.tools.datetimes`

We were using it in nextstrain/augur's parse to get information about the datetime resolution. Currently seeing how to migrate to pandas v2 given the removal.