Use strict = True for mypy_bootstrap.ini

[sobolevn]

We almost had all strict flags set anyway, so many it will just work.
If not, it would be easier to turn something off.

[github-actions]

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

[sobolevn]

Thanks for the quick review!

[davidhalter]

On a related note: Is it intentional that warn_unreachable is not used? I have recently realized that this is not turned on and it feels a bit strange. There are about 39 errors when running with --warn-unreachable (on mypy_self_check.ini) and it brings up a few questionable things like https://github.com/python/mypy/blob/master/mypy/test/testpep561.py#L180.

It is also questionable why --strict does not just enable --warn-unreachable. A lot of people probably think "I'm running --strict, there's nothing else important to enable". However that's an entirely different topic.

[sobolevn]

We should add --warn-unereachable to --strict in 2.0
@davidhalter can you please open two new issues about these two topics?

[davidhalter]

@sobolevn Done, see the issues above.