Fix for mypy 1.4 changes

[VincentVanlaer]

This PR contains all the necessary changes to the plugin, tests and stubs for mypy 1.4

Api changes

• See https://github.com/python/mypy/pull/14872 and https://github.com/python/mypy/pull/13546 for API changes
• Tests have been updated as the output format of mypy has changed a bit

Stubtest changes

As stubtest is part of mypy, and therefore also got updated, the stubs required signficant changes:

List of relevant changes:

• A whole bunch of types that are final at runtime, are now final in for type checking purposes as well
• Add missing attrs decorators to some types. Internal attrs members have been added to the allowlist
• StrictFIFOLock is no longer a subclass of Lock, as Lock is suposed to be final.
• Converted attributes to read-only properties whenever this was necessary to match trio

Minor changes:

• Ignore internal APIs and tests (previously seemed to be the case by default)
• Typevars and aliases have been prefixed with underscores, so that they are not part of the public interface of the modules (as they are not available at runtime)

Fixes #84 Fixes #85

[A5rocks]

Could you check the mypy version we are passed to choose whether to pass default= or not?

I don't like having to drop mypy < 1.4

[VincentVanlaer]

I have added a check for the mypy version (and some other required changes due to changes in trio)

[VincentVanlaer]

Thanks for all the reviews!