Check all our python scripts with mypy

Fix a couple of mypy errors and add a mypy config file to deal with the
construct module, to reach the point where all our scripts pass mypy
default checks.

Check all scripts with mypy in the Makefile `make check'.

[Backport commit 7cc3f6ca and add mypy to the CI files.]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>