Respect sys.platform platform check assertion inside functions

[nipunn1313]

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()

[AlexWaygood]

I figured out a hacky workaround for this in #11909. Instead of having assert sys.platform == "win32" at the top of the function, instead do this:

if sys.platform != "win32":
    assert False

[ncoghlan]

Just noting that the assert False form also works when filtering out single code branches within a larger function using a library or application specific check. For example:

_WINDOWS_BUILD = hasattr(os, "add_dll_directory")

def some_function():
    if _WINDOWS_BUILD:
        if sys.platform != "win32":
            assert False
        # mypy allows Windows-only API usage here
    else:
        if sys.platform == "win32":
            assert False
        # mypy allows *nix-only API usage here