drop python<=3.7 support

[kloczek]

According to https://endoflife.date/python python 3.7 has been EOSed 27 Jun 2023. Filter all code over pyupgracde --py38-plus.

TODO:

• [ ] Write new tests or update the old ones to cover new functionality.
• [ ] Update doc-strings where appropriate.
• [ ] Update or write new documentation in packit/packit.dev.
• [ ] ‹fill in›

Fixes

Related to

Merge before/after

RELEASE NOTES BEGIN

Packit now supports automatic ordering of ☕ after all checks pass.

RELEASE NOTES END