check-sr-results: stricter typing

Add more type annotations to check-sr-results and rework some bits to make
mypy happy.

With that, we have no more exception in .mypy.ini and all our scripts are
checked with mypy in strict mode.

[Backport commit 784c5e34 ("check-sr-results: stricter typing").]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>