Re-add support for Python 3.7 and 3.8.

[gdiaz384]

As per https://peps.python.org/pep-0563/

I certify: https://developercertificate.org/

The above changes and any subsequent changes are made under the same license as the original project which is MIT. A copy of this license is available at: https://github.com/gdiaz384/ass_tag_parser/blob/master/LICENSE.md

I have not tested the changes on Python 3.9+ as that does not relate to my use case, but the changes are unlikely to break anything.