Remove support for Python 3.9

[ml-evs]

Closes #2149

• Changes default Python version to 3.10
• Updates all annotations to use "T | T'" syntax, especially "T | None" replacing Optional[T]
• Modifies type adapter to correctly resolve new optional syntax for OPTIMADE schema types

Note that the JSON schema rep was unchanged here, so it might be beneficial to rewrite the type inference stuff to target the JSON schema directly in future (i.e., parsing the various anyOf bits when dealing with union types/optional fields).

EDIT: I'm going to merge this once the tests pass, but not sure if you'd want to give a cursory look at e814cd26305f8cd7e1f604e5b2462e1dc1328d55 @CasperWA -- essentially, switching all the model annotations to T | None-style caused the match with your custom UnionType to fail. I fiddled around a bit with the other types you defined, but seems like the constructs like the official types.UnionType don't exist in those cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.80%. Comparing base (4aaf1ff) to head (e814cd2).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2158      +/-   ##
==========================================
+ Coverage   90.79%   90.80%   +0.01%     
==========================================
  Files          75       75              
  Lines        4824     4819       -5     
==========================================
- Hits         4380     4376       -4     
+ Misses        444      443       -1     

Flag	Coverage Δ
project	90.80% <100.00%> (+0.01%)	⬆️
validator	90.80% <100.00%> (+0.01%)	⬆️

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