Declare support for Python 3.10 and update links

[hugovk]

Python 3.10 is due out in just over a couple of weeks, and is already tested by the CI. Let's add the Trove classifier.

Also in setup.cfg update the url to the new, redirected domain, and add the changelog to the project URLs; a common task when visiting PyPI is to check what changed in the most recent release.

• Shall I also PR these link updates to the rewrite branch?
• Does this need a news file?

Thanks for contributing a pull request!

If you are contributing for the first time or provide a trivial fix don't worry too
much about the checklist - we will help you get started.

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• [n/a] added/updated test(s)
• [n/a] updated/extended the documentation
• [n/a] added relevant issue keyword
  in message body
• added news fragment in changelog folder
  • fragment name: <issue number>.<type>.rst for example (588.bugfix.rst)
  • <type> is must be one of bugfix, feature, deprecation, breaking, doc, misc
  • if PR has no issue: consider creating one first or change it to the PR number after creating the PR
  • "sign" fragment with -- by :user:`<your username>`.
  • please, use full sentences with correct case and punctuation, for example:

    Fixed an issue with non-ascii contents in doctest text files -- by :user:`superuser`.

  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[gaborbernat]

[asottile]

setup-cfg-fmt will do this automatically once 3.10 final is available (as you can see it undid your change) -- the intent is that you don't need these types of PRs (which I think are noisy!)