Refactor/310 plus typehints

[jlnav]

With Python 3.9 dropped, many type hints can be modernized, e.g:

Optional[Union[int, float]] -> int | float | None

Changes were guided by flake8-modern-annotations and flake8-type-checking,
which were added to the dev environment.

[codecov]

Codecov Report

Attention: Patch coverage is 98.58156% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.36%. Comparing base (f181c1a) to head (a51fd9c).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
libensemble/executors/balsam_executor.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1522      +/-   ##
===========================================
- Coverage    78.37%   78.36%   -0.02%     
===========================================
  Files           76       76              
  Lines         7696     7678      -18     
  Branches      1144     1144              
===========================================
- Hits          6032     6017      -15     
+ Misses        1468     1466       -2     
+ Partials       196      195       -1     

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[jmlarson1]

Is ipdb now a dependency?

[jlnav]

Nope, I just added it to the dev virtual environment.