Drop support for EOL Python 3.7

[hugovk]

Changes proposed in this pull request:

• Drop support for Python 3.7, EOL this month
  • https://devguide.python.org/versions/
  • https://peps.python.org/pep-0537/
• Not planning a major bump, requires-python = ">=3.8" makes sure people get the right version

[codecov[bot]]

Codecov Report

Merging #114 (f28d22b) into main (e76580b) will decrease coverage by 3.85%. The diff coverage is n/a.

@@             Coverage Diff             @@
##              main     #114      +/-   ##
===========================================
- Coverage   100.00%   96.15%   -3.85%     
===========================================
  Files            1        1              
  Lines           78       78              
===========================================
- Hits            78       75       -3     
- Misses           0        3       +3     

Flag	Coverage Δ
macos-latest	?
ubuntu-latest	96.15% <ø> (ø)
windows-latest	?

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[hugovk]

Continued in #115.