Partial backport of #19176.
Closes #19310.

Replace all NotImplemented-based runtime placeholders with typing.Any. The problem with the previous
approach is that typing lacks the necessary special-casing for NotImplemented, and will thus raise an exception
when used with subscriptable typing constructs (e.g. Union).

Note that the relevant issue is limited to python 3.7 installations that lack the typing-extensions package.