Require Python 3.8+.

[bdice]

Description

NEP 29 has dropped support for Python < 3.8. This PR updates signac-flow to require Python 3.8+.

See also glotzerlab/signac#715.

I also updated our Jinja requirement to 3.0.0. Older versions of Jinja are incompatible with the latest version of markupsafe, one of Jinja’s dependencies. Requiring Jinja 3.0.0+ fixes this, and allows us to drop some compatibility code.

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 #628 (5d95deb) into master (0ee68b1) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
+ Coverage   79.20%   79.29%   +0.09%     
==========================================
  Files          33       33              
  Lines        3231     3227       -4     
  Branches      688      686       -2     
==========================================
  Hits         2559     2559              
+ Misses        529      527       -2     
+ Partials      143      141       -2     

Impacted Files	Coverage Δ
flow/project.py	81.63% <100.00%> (+0.20%)	⬆️

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[csadorf]

Looks good!