Make support/python/mkdocstrings_handlers/cxx/__init__.py PEP 8 com…

…pliant (2 of 2) (#4115)

* Change indents to 4 spaces.

* Run isort.

* Remove one extra space on the left hand side of each assignment at p.communicate's input.

* Remove 'from __future__ import annotations'.
This requires changing a 'list[]' into a 'List[]'.

We had previously added this import because the code was making use of operator '|'.
But that is no longer true, so the import shouldn't be needed.