Add --strict-bytes flag to disable promotions to bytes

[JukkaL]

Feature

We currently have the undocumented flags --disable-bytearray-promotion and --disable-memoryview-promotion that turn off promotions from bytearray/memoryview to bytes. These are a bit unwieldy, and I'm proposing that we add a new flag called --strict-bytes that enables both of these flags, and make it an officially documented feature. In mypy 2.0 we'd switch --strict-bytes on by default.

Additional details:

• This should be documented.
• Also support --no-strict-bytes for the inverse. This allows users to prepare for mypy 2.0 even if they aren't ready to turn on the flag.
• This probably shouldn't be a per-module option, since migration to the new behavior appears to be fairly easy even for large codebases.
• We'll still keep the old undocumented flags for now, but they might be removed in the future.

Pitch

Currently bytearray and memoryview are assignable to bytes, which is incorrect, but this has been tricky to fix mostly due to stub quality. Stubs seem to have improved enough for this to be practical now -- any anyway switching the default would give more incentives to fix stubs.

The new flag is shorter and easier to remember, and it combines both of the features so it's simpler. The naming is also arguably consistent with --strict-equality and friends. In my experience dropping the memoryview promotion rarely causes trouble, so having it as a separate flag doesn't seem important.

[JukkaL]

Anybody want to work on this? We'd probably need a feature release with this available before we can release mypy 2.0, due to our backward compatibility policy.

[wyattscarpenter]

How about enabling --strict-bytes in --strict in the meantime? Seems to me like that wouldn't require a major version release, based on the warning that --strict behavior may change. It also makes sense, and is very easy to do.

Additionally, #19049 shows only 6 diffs on mypy-primer, with an average of only a couple new errors each, so I think this would be a manageable burden to add to the process of upgrading mypy on a minor version.