Add --debug-serialize option

[cdce8p]

Currently, mypy_primer sets --cache-dir=/dev/null which disables cache generation. This can result in errors being missed which would normally come up during tree.serialize(). Removing --cache-dir=/dev/null isn't practical.

This PR adds a new debug / test option --debug-serialize which runs tree.serialize() even if cache generation is disabled to help detect serialize errors earlier.

Refs

• AssertionError with nested TypeVar #14137
• Replace types-attrs with attrs hauntsaninja/mypy_primer#54 (review)

cc: @hauntsaninja

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[cdce8p]

@ilevkivskyi Would you mind taking a quick look?
This option would have helped catch crashes like the one in #14137 much earlier with the mypy_primer.

[cdce8p]

@ilevkivskyi Anything else that needs to be done here or can this be merged?
The nexts steps will be

• Modify the primer to allow passing additional arguments directly to mypy
• Update the CI workflow to pass --debug-serialize

[AlexWaygood]

• Modify the primer to allow passing additional arguments directly to mypy

That one's already been completed: hauntsaninja/mypy_primer@ceecd71