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 #118 (75be1e4) into main (8619d27) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 75be1e4 differs from pull request most recent head a071260. Consider uploading reports for the commit a071260 to get more accurate results

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
- Coverage   99.06%   99.05%   -0.02%     
==========================================
  Files           9        9              
  Lines         745      737       -8     
==========================================
- Hits          738      730       -8     
  Misses          7        7              

Flag	Coverage Δ
macos-latest	97.82% <100.00%> (-0.03%)	⬇️
ubuntu-latest	97.82% <100.00%> (-0.03%)	⬇️
windows-latest	95.92% <100.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
src/humanize/__init__.py	100.00% <100.00%> (ø)
src/humanize/i18n.py	98.21% <100.00%> (-0.15%)	⬇️