Add support for Python 3.10, drop EOL 3.5, replace Travis CI with GitHub Actions

[hugovk]

• Add support for Python 3.10, due out in a couple of weeks: https://www.python.org/dev/peps/pep-0619/
• Drop support for EOL Python 3.5.
• Travis CI is being turned off for the pytest-dev org (internal discussion), due to the recent security handling (1, 2), so replace CI with GitHub Actions
• This lets us also easily test on macOS and Windows!
• Also update some URLs from old https://github.com/ImXron/pytest-stress to new https://github.com/pytest-dev/pytest-stress

[hugovk]

Example build:

https://github.com/hugovk/pytest-stress/actions/runs/1256940560

[hugovk]

Thanks for doing this! Feel free to squash/merge when you're ready.

You're welcome! I don't have merge rights here, but I'm otherwise ready.

Do we also need to remove Travis from this package's settings?

If you mean these required status checks, yes, these'll need removing (and optionally new GHA ones adding after merge):

If you mean disabling Travis from this repo, no need, @RonnyPfannschmidt will be disabling it from the whole pytest-dev org at some point:

• https://github.com/orgs/pytest-dev/teams/contributors/discussions/5