Use type alias for temporal intervals

[duckontheweb]

Related Issue(s):

• Closes Type issues with TemporalExtent #586

Description:

Adds 2 type variables that are used in the TemporalExtent class:

• TemporalIntervals - This is used for properly formatted STAC temporal intervals (i.e. a list of lists)
• TemporalIntervalsLike - This is used for the intervals argument to TemporalExtent where we allow either a list of lists or a list of optional datetimes and convert it into a list of lists.

I tried using a covariant TypeVar as I had initially suggested in #586 (comment), but that did not fix the type errors. Using a Union type of all the possible combinations seemed to be the only way to make mypy happy.

Also note that this adds a test whose only purpose is type checking when we run mypy. If there are suggestions on a better place to put this kind of check I'm happy to change that.

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.

[duckontheweb]

cc: @cuttlefish

[codecov-commenter]

Codecov Report

Merging #744 (0a026b4) into main (4b30217) will increase coverage by 0.00%.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##             main     #744   +/-   ##
=======================================
  Coverage   94.33%   94.34%           
=======================================
  Files          77       77           
  Lines       11285    11291    +6     
  Branches     1347     1347           
=======================================
+ Hits        10646    10652    +6     
  Misses        459      459           
  Partials      180      180           

Impacted Files	Coverage Δ
pystac/collection.py	94.26% <80.00%> (+0.04%)	⬆️
tests/test_collection.py	100.00% <100.00%> (ø)

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[gadomski]

I did have one question about the type union but I'm probably not understanding it correctly so approving so I don't block if I'm wrong :-).