Optionally run mypyc on projects

[zsol]

This PR

• allows Projects to run a custom command to verify if mypyc breaks on the project.
• turns this feature on for Black

[hauntsaninja]

Thanks for working on this! Left two quick comments.

Do you have a sense of how long this takes? Might need to be able to flag this off to avoid slowing down typeshed CI (which won't benefit from this)

[zsol]

For Black this takes around 6 minutes on my laptop. Definitely worth gating off for typeshed. I can add a command line flag if that helps

[hauntsaninja]

A command line flag would be great!

[zsol]

I've addressed your comments in the new commit, going to add a command line flag now. Thanks for the blazing fast reviews :)

[mr-c]

Ping; it would be great to have this for schema-salad and cwltool given the mypyc regressions we experienced in the last two releases