ci: No longer test latest packages on Python 3.8

[b-butler]

Description

Oldest tests still remains on Python 3.8. NumPy no longer supports Python3.8 in their latest packages.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[codecov]

Codecov Report

Merging #938 (d68c270) into main (9f96a64) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #938   +/-   ##
=======================================
  Coverage   85.68%   85.68%           
=======================================
  Files          20       20           
  Lines        3458     3458           
  Branches      758      758           
=======================================
  Hits         2963     2963           
  Misses        335      335           
  Partials      160      160