Add: refinement of ReadOnly TypedDict fields in subclasses

[ldeld]

Fixes #7435

Adds the possibility to refine read-only keys in TypedDicts through inheritance.

The refinement is limited to read-only keys because refining mutable keys leads to unsafe edge cases (see #7435 (comment))

The rules are straight-forward:

• The original key type and refined type must both be read only
• The refined type must be a subtype of the original

@hauntsaninja, I saw you were open to receiving a PR for this. This is the first time I look at mypy's source code, so I apologize if the code is not up to standard! (I was particularly unsure if it is OK to modify failure messages.)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

django-stubs (https://github.com/typeddjango/django-stubs): 2.07x faster (51.1s -> 24.6s in a single noisy sample)

[ilevkivskyi]

Unfortunately, it is not safe to call is_subtype() during semantic analysis (except in semanal_typeargs.py which is the last pass, but this is not helpful for you). Some types may not be fully analyzed and this will cause all kinds of weird behaviors.

This kind of checks can be only done during type-checking phase.