Drop support for Python 3.9 and modernize the codebase

[mattpap]

This PR drops support for Python 3.9 and takes advantage of the following PEPs: 634 (pattern matching), 604 (A | B), 613 (TypeAlias), 647 (TypeGuard). I converted a few obvious cases from if-elif-else to match-case as a trial run. I tenatatively added this PR to 3.4 milestone, but I suppose we may want to postpone this till 3.5.

fixes #13515

[codecov]

Codecov Report

Attention: Patch coverage is 90.57971% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 92.60%. Comparing base (8a5a35e) to head (f6dacf0).
Report is 18 commits behind head on branch-3.5.

Additional details and impacted files

@@              Coverage Diff               @@
##           branch-3.5   #13634      +/-   ##
==============================================
+ Coverage       92.59%   92.60%   +0.01%     
==============================================
  Files             326      325       -1     
  Lines           20735    20732       -3     
==============================================
  Hits            19199    19199              
+ Misses           1536     1533       -3     

[bryevdv]

This is a nice minpy-bump, as they go. NEP 29 data for dropping Python 3.9 is Apr 05, 2024 so Bokeh 3.5 is probably ideal.

[mattpap]

NEP 29 data for dropping Python 3.9 is Apr 05, 2024 so Bokeh 3.5 is probably ideal.

Our BEP 9 says it should be at least 3 months after the most recent Python release, which is about now. With pushing this to 3.5 I was thinking about panel.

[philippjfr]

+1 on waiting until 3.5. That said I would suggest that we amend BEP 9 to align with NEP 29 if they are indeed not synced. I would consider NEP 29 quite aggressive and would not want to be out ahead of them.

[bryevdv]

@philippjfr we can revisit, but the original rationale for the difference from NEP-29 is here #10558 (comment) It's possible NEP-29 has also been updated since then too.

[bryevdv]

@mattpap I'd suggest merging this sooner rather than later in order to shave some work off our CI pipeline

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