Require Python 3.8+, update CI.

[bdice]

Description

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

See also related changes in glotzerlab/signac#715

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.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.36%. Comparing base (310a0ca) to head (d725f82).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #120   +/-   ##
=======================================
  Coverage   63.36%   63.36%           
=======================================
  Files          18       18           
  Lines         666      666           
=======================================
  Hits          422      422           
  Misses        244      244           

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[csadorf]

Looks good!