Remove 3.8; updating annotation typing

[CodyCBakerPhD]

Pre-PR to #507

Removes 3.8 and updates the relevant typing to 3.9+ standard (using built-in references instead of typing module classes)

[stephprince]

Looks good! Updated a couple extra files

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.97%. Comparing base (c7861f4) to head (e459c16).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/nwbinspector/_dandi_inspection.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #508      +/-   ##
==========================================
+ Coverage   79.53%   84.97%   +5.44%     
==========================================
  Files          48       48              
  Lines        1412     1411       -1     
==========================================
+ Hits         1123     1199      +76     
+ Misses        289      212      -77     

Files with missing lines	Coverage Δ
src/nwbinspector/_configuration.py	96.22% <100.00%> (ø)
src/nwbinspector/_formatting.py	85.40% <100.00%> (ø)
src/nwbinspector/_nwb_inspection.py	83.82% <100.00%> (+4.41%)	⬆️
src/nwbinspector/_organization.py	100.00% <100.00%> (ø)
src/nwbinspector/testing/_testing.py	83.63% <100.00%> (+1.81%)	⬆️
src/nwbinspector/tools/_dandi.py	20.68% <100.00%> (ø)
src/nwbinspector/utils/_utils.py	91.89% <100.00%> (ø)
src/nwbinspector/_dandi_inspection.py	98.03% <90.00%> (+76.88%)	⬆️

... and 2 files with indirect coverage changes