chore: remove all uses of deprecated APIs and make mypy report an error for use of a deprecated API

[ThosRTanner]

Depends on #823

Why is this needed?

We shouldn't be using deprecated APIs...

Proposed Changes

• Use non-naive datetime objects in Lease.py (and change the version number of Lease objects).
• Don't pass a utcnow method to the constructors, but use mocking to control behaviour.
• Replace logger.warn with logger.warning
• Enable deprecated error in mypy.
• removed regex from .coveragerc because it's now unnecessary and appears to cause latest coverage to misfire.

Does this PR introduce any breaking change?

Sort of. It changes the version number of the Lease objects (from recipes/lease.py) as the timestamps used are no longer naive, and additionally makes them no longer take an argument to the constructor for creating a datetime object. Those parameters were defined as being only for test, and now we use freezegun to manipulate dates and times, so they're no longer needed.

Codecov note

codecov flagged an uncovered changed line, for changing logger.warn to logger.warning in an untested path (currently untested because it'd require zookeeper to be doing something odd). I added a suppression for this as I think unit tests for this would be fragile.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.01%. Comparing base (f54c4fd) to head (603ccb7).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #825      +/-   ##
==========================================
+ Coverage   94.90%   96.01%   +1.10%     
==========================================
  Files          27       27              
  Lines        3811     3739      -72     
==========================================
- Hits         3617     3590      -27     
+ Misses        194      149      -45     

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