Upgrade mypy to 0.971 or newer

[DMRobertson]

We have currently locked mypy 0.950. Since then, 0.960 and 0.971 have been released. I am particularly interested by ParamSpec-related fixes and improvements to the stdlib stubs in typeshed.

Now that a sufficiently new version of mypy-zope is available, we should upgrade mypy and mypy-zope to take advantage of the latest improvements.

[DMRobertson]

WIP at https://github.com/matrix-org/synapse/tree/dmr/mypy-upgrade

[DMRobertson]

There were a bunch of failures on other PRs (but none on #13521 itself?). Needs investigation later.

[DMRobertson]

0.981 is now available.

[DMRobertson]

There were a bunch of failures on other PRs (but none on #13521 itself?). Needs investigation later.

I suspect something went wrong with mypy's cache. python/mypy#10221 looks related, and also mentions mypy-zope. How suspicious!!