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 #166 (8dbb146) into main (ed37dd1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #166   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           16        13    -3     
=========================================
- Hits            16        13    -3     

Flag	Coverage Δ
macos-latest	100.00% <100.00%> (ø)
ubuntu-latest	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

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

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