Name already in use

- Use the new `allow-prereleases` key for `actions/setup-python`. If we set this to `true`, then we can just use `3.12` in our CI matrix, instead of `3.12-dev`. That means we won't have to worry about updating the matrix from `3.12-dev` to `3.12` when 3.12.0 comes out in the autumn; our CI will automatically start using the latest version of Python 3.12.
- Colorise the output of flake8.
- Don't bother upgrading pip in CI. It's not really necessary, and wastes a few seconds. Disable the "you're using an older version of pip" message instead, using the `PIP_DISABLE_PIP_VERSION_CHECK` environment variable.

#178)

For compatibility with Python 3.12, which adds this undocumented
parameter along with the existing covariant/contravariant ones.
Intentionally leaving this undocumented: it's here in case we want
to make variance on ParamSpec mean something later, but it currently
has no supported use.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Make our TypeAliasType behave exactly like the 3.12 one

I started out trying to backport python/cpython#104571, but
realized that it makes sense to backport CPython's whole TypeVarTests
class since we now have our own implementation of TypeVar.

I dropped test_var_substitution and test_bad_var_substitution
since they rely on the internal __typing_subst__ method, and the
type substitution logic is generally very hard to get precisely
the same across versions.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

…hon 3.12 (#163)

…time_checkable` and various runtime-checkable protocols (#143)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Followup to #150

Co-authored-by: AlexWaygood <alex.waygood@gmail.com>