[MNT] Remove upper bound on python_requires

[matthewfeickert]

Description

• Remove upper bound of Python 4 on python_requires as advocated for in Henry Schreiner's blog post "Should You Use Upper Bound Version Constraints?".

Motivation and Context

This change follows best practices for libraries and frameworks as generally recommended by the PyPA.

Resolves #780

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.

[bdice]

Note: the same thing should also be done for additional glotzerlab repositories.

signac-flow, signac-dashboard, rowan, gsd (uses ~=3.6 which would not match 4), garnett, hoomdpy-addons.

I can help with signac packages but leave the rest to other maintainers. cc: @b-butler @joaander @vyasr

[tommy-waltmann]

Just one quick comment.

[vyasr]

@matthewfeickert thanks for the contribution!

[vyasr]

Thanks again.