Run mypy on tests

[jacobtomlinson]

In order to work on #491 we are going to need to be able to test our type annotations. Currently tests are excluded from mypy. This PR adds them back in again.

There were a few failures, many related to the sync API annotations highlighted in #491. For now I've just removed the annotation from the test signature so that mypy skips as we do not enforce strict type checking yet (xref #428).

This PR should merge cleanly with no mypy failures, but will allow us to gradually introduce typing into the tests and therefore test the types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.19%. Comparing base (87063fc) to head (f13d7e0).
Report is 114 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
+ Coverage   94.61%   95.19%   +0.57%     
==========================================
  Files          29       30       +1     
  Lines        3141     3973     +832     
==========================================
+ Hits         2972     3782     +810     
- Misses        169      191      +22     

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