Check support of Mypy's fixed format cache

[sureshjoshi]

I don't know if this requires any changes to how Pants handles mypy caches.

https://github.com/python/mypy/blob/master/CHANGELOG.md#fixedformat-cache-experimental

[cburroughs]

I don't know if this requires any changes to how Pants handles mypy caches.

I can answer that it will.

[sureshjoshi]

Well, nuts...

I've got a PR for 1.18.2 coming up momentarily - doesn't add that field, but was hoping to get the perf gain.

Our mypy code is pretty messy and supports really far back. Might be worth re-evaluating that too.

[sureshjoshi]

@cburroughs Do you have any more information on this? A TLDR on what changes we need to support? Just locations or anything more substantial?

[cburroughs]

We would need something like #16290 to the new format (or maybe the new format is already safe? so we need a way to opt out of that) and then add some sort of mypy-cache-format={none,sqlite,new-thing} flag.