Test more PyPy

[altendky]

Draft for:

• pywin32 is not available for PyPy. This likely means we need to just not support the serial extra on PyPy on Windows. Anything else?

Scope and purpose

Add a few words about why this PR is needed and what is its scope.

Add mentions of things that are not covered here and are planed to be done
in separate PRs.

Remove this section

Please have a look at our developer documentation before submitting your Pull Request.

Please note that the trac ticket, news fragment, and review submission portions of this process apply to all pull requests, no matter how small; if you don't do them, it's likely that nobody will even notice your PR needs a review.

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• I have updated the automated tests.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.

[altendky]

Kick CI

[altendky]

Kick CI to try out pypa/virtualenv#2072.