Invalidate mypy cache if config changes

[cdce8p]

Implement Plugin.report_config_data to return all plugin config data. For every run, mypy will compare if the data matches the cached one if not the cache is discarded.

report_config_data is called multiple times for every file. I would have liked to use cached_property but that requires 3.8, unfortunately.

https://github.com/python/mypy/blob/v0.991/mypy/plugin.py#L539

This is my first PR here, so I wasn't quite sure which branch to target as this is an issue in both 1.10.x and main. Please let me know if I should change anything and I'll do so.

[samuelcolvin]

This is great, thanks so much.

Please change the base to main, if we think it should be backported to v1.10, we can do that easily.

If you want it backported, please also add a change description.

[cdce8p]

Please change the base to main, if we think it should be backported to v1.10, we can do that easily.

If you want it backported, please also add a change description.

Backporting it later as well would be great. I can open a separate PR for it after this one is merged.

The change description in HISTORY.md? Should I add it here already or only for the backport PR?

[samuelcolvin]

No, see https://github.com/pydantic/pydantic/tree/main/changes

We have a bot to backport.

[samuelcolvin]

thanks so much.