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]

Codecov Report

Merging #245 (ac0f313) into main (54ccf40) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
- Coverage   94.83%   94.83%   -0.01%     
==========================================
  Files           5        5              
  Lines        2364     2361       -3     
==========================================
- Hits         2242     2239       -3     
  Misses        122      122              

Flag	Coverage Δ
macos-latest	94.79% <100.00%> (-0.01%)	⬇️
ubuntu-latest	94.79% <100.00%> (-0.01%)	⬇️
windows-latest	94.74% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/prettytable/__init__.py	100.00% <100.00%> (ø)

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