Remove Python 3.7 support

[duckontheweb]

Related Issue(s):

• Closes Drop support for python 3.7 in 1.6.0 #814

Description:

Drops support for Python 3.7 according to our Python deprecation schedule.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #853 (b21d119) into main (ccead6b) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
+ Coverage   94.30%   94.32%   +0.01%     
==========================================
  Files          83       83              
  Lines       11967    11963       -4     
  Branches     1404     1403       -1     
==========================================
- Hits        11286    11284       -2     
+ Misses        497      496       -1     
+ Partials      184      183       -1     

Impacted Files	Coverage Δ
pystac/summaries.py	79.54% <ø> (+0.65%)	⬆️

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