* 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.
* Make support/python/mkdocstrings_handlers/cxx/__init__.py PEP compliant.
* Rollback minor change in __init__ signature.
* Rollback minor change in __init__ signature.
* Fix previous commit.
* Add 'from __future__ import annotations' to fix Python 3.8 error when using operator '|'.
* Change doxyxml2html([n]) to doxyxml2html(list(n)) as suggested by Victor.
Change convert_type return type to Optional[str].
