Fix python version in tests (plus additional minor fixes)

[davidwilby]

Whilst looking into #115 I noticed that something funky was going in the CI environment when running tests in which a different python version from the ones specified in the version matrix, using python 3.10 for all of the tests.

This turned out to be because of the step in which space was being cleared, inadvertently deleting the python executable installed by the setup-python action at /opt/hostedtoolscache. I've replaced this with a different option using the jlumbroso/free-disk-space action for ubuntu runners. Have tested this out over on my fork and confirmed that the expected python versions are being used in running tests by tox.

Additionally, this PR:

• upgrades some deprecated actions versions,
• Fixes a bug found when running tests with python 3.8 in which the post-3.8 typehint style was being used, resulting in an error. I suggest this fix rather than shifting to the newer style since python 3.8 is still technically in support for the time-being. (Edit: https://pypi.org/project/typing-extensions/ would be another option for this, but maybe in a separate PR)

[tom-andersson]

Great catches @davidwilby. Fairly shallow review but passes a sanity check of what you've described. LGTM (and sorry for the delay).

[tom-andersson]

@all-contributors please add @davidwilby for maintenance

[allcontributors]

@tom-andersson

I've put up a pull request to add @davidwilby! 🎉