Respect sys.platform platform check assertion inside functions

This is a feature request

From the docs
https://mypy.readthedocs.io/en/stable/common_issues.html#python-version-and-system-platform-checks

More specifically, mypy will understand the use of sys.version_info and sys.platform checks
within if/elif/else statements.

and

As a special case, you can also use one of these checks in a top-level (unindented) assert; this makes mypy skip the rest of the file.

It would be nice if this kind of code would also typecheck on mac/linux.
Currently the option is to refactor such that you can define the entire function within an if sys.platform == "win32"

class A:
   def func_on_win_only() -> str:
      assert sys.platform == "win32"
      return sys.getwindowsversion()

Start by reproducing the example on a non-Windows platform and read the linked mypy documentation on sys.platform checks. Trace the existing handling for top-level assertions, then add regression coverage for an assertion inside a function and verify that the Windows-only call type-checks on macOS or Linux.