Mypy and pylint was previously not included in TESTING.md.
Also, installation of pre-commit, pytest and pytest-cov outside
of a virtual environment was not detailed.

CONTRIBUTIONS.md had an old Python version listed in the conding standard section.

Signed-off-by: erik.andersson@arm.com <erik.andersson@arm.com>
Change-Id: Idff9454083e41d719e6d75e90cb2be2861500eb9