Updates Python linters, spellchecking, and ShellCheck versions. The PR links are updated for
the dependency versions in test/README.md. ShellCheck SC2230 removed to align with with new
behaviour in v0.7.1.
Fixes#19346.
Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."
Mypy is used in lint-python.sh to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.
Useful resources:
* https://docs.python.org/3.5/library/typing.html
* https://www.python.org/dev/peps/pep-0484/
Primarily I'd like to remove this because it is very imprecise, due to
Python's dynamic nature, giving it a large list of false positives that
need to be listed as exceptions. See for example #16906.
It's also a frequent source of complaints. I'm doubtful of the
usefulness of checking for dead code in a linter in the first place.
Having some dead code in the test framework for a while is not a
disaster.
