Updates for recent mypy

[PeterJCLaw]

This picks up a change to mypy's defaults to prefer explicit Optionals and python/typeshed#8542 meaning one of our type: ignores is no longer needed.

[PeterJCLaw]

The Python 3.6 failure looks like it's due to recent mpy having dropped support for Python 3.6, meaning that we're getting an older version which doesn't have the typeshed fix.

Solutions include not type checking on 3.6, grabbing a custom typeshed on 3.6 (kinda complicated but possible) or just dropping support for 3.6.

[attwad]

Let me update the supported versions, I'll add more recent ones and remove 3.6

[attwad]

alright can you take the latest changes from master? 3.6 should be out and 3.10, 3.11 should be added now.

[PeterJCLaw]

I've put up #163 to fix CI on Python 3.10 (currently interpreted as 3.1) and rebased this on top of that.
The failures remaining are:

• collections.Iterable import
• pygame not installing on 3.11

[PeterJCLaw]

Hrm, this is complicated by what appears to be a pygame bug. I think we can work around that, but an alternative quicker solution to fix master may be to pin mypy to an older working version (0.971 should I think work).

[PeterJCLaw]

@attwad sorry this has been a bit iterative; I thiiink that with #164 (on which this PR is based) and this PR together the CI should now be sorted. There's a small code change here to help mypy, but I hope not a controversial one.

[attwad]

Thank you for the perseverance :)