Remove upper bound on python_requires.

[bdice]

Description

See glotzerlab/signac#781.

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 #654 (166200e) into master (38b176c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #654   +/-   ##
=======================================
  Coverage   79.06%   79.06%           
=======================================
  Files          33       33           
  Lines        3238     3238           
  Branches      687      687           
=======================================
  Hits         2560     2560           
  Misses        536      536           
  Partials      142      142           

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[b-butler]

👍