Flip fixed-format cache to on by default

[ilevkivskyi]

Since this is scheduled for v1.20 it is better do this sooner than later. So that people who are using mypy master can give some early feedback.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Let's do it! Maybe we should also enable sqlite cache by default (in 1.20), for even more incremental mode performance gains?

[ilevkivskyi]

@JukkaL

Maybe we should also enable sqlite cache by default (in 1.20), for even more incremental mode performance gains?

Maybe. I am thinking maybe we should also add some helper script to extract cached content from DB, or do we already have one?

[ilevkivskyi]

Btw, I just noticed that some of the scripts (like convert-cache.py, diff-cache.py, apply-cache-diff.py) will not work with fixed format. The first one is probably easy to fix, the other two are less trivial.