improve typing for `await Band.objects().first()`

[dantownsend]

Related to https://github.com/piccolo-orm/piccolo/issues/561

When doing this query:

band = await Band.objects().first()

MyPy now knows the response is t.Optional[Band]. This was possible by using Generic and TypeVar from Python's typing module.

[codecov-commenter]

Codecov Report

Merging #562 (5690fa1) into master (aee9388) will increase coverage by 0.05%. The diff coverage is 96.01%.

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
+ Coverage   91.25%   91.30%   +0.05%     
==========================================
  Files         107      108       +1     
  Lines        7317     7383      +66     
==========================================
+ Hits         6677     6741      +64     
- Misses        640      642       +2     

Impacted Files	Coverage Δ
piccolo/apps/asgi/commands/new.py	73.43% <ø> (ø)
piccolo/query/mixins.py	94.59% <ø> (-0.08%)	:arrow_down:
piccolo/query/methods/select.py	97.90% <91.07%> (-1.70%)	:arrow_down:
piccolo/query/base.py	87.50% <93.75%> (+0.80%)	:arrow_up:
piccolo/query/proxy.py	94.28% <94.28%> (ø)
piccolo/apps/migrations/commands/clean.py	93.93% <100.00%> (ø)
piccolo/custom_types.py	100.00% <100.00%> (ø)
piccolo/engine/postgres.py	91.28% <100.00%> (ø)
piccolo/engine/sqlite.py	92.08% <100.00%> (ø)
piccolo/query/methods/alter.py	90.12% <100.00%> (+0.04%)	:arrow_up:
... and 10 more

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[lgtm-com[bot]]

This pull request introduces 3 alerts when merging 0194ff9f88a577f3e9467a2514db61fe2e813224 into cacf7c507f20d153bd40306bf250816e6edb0512 - view on LGTM.com

new alerts:

• 3 for First argument to super() is not enclosing class

[Bobronium]

I propose to use TypeVar("ClassT", bound="Class") instead of Self, since Self is relatively new and not supported by mypy/pycharm, afaik.

[dantownsend]

Remaining tasks:

• [x] There's a small bug causing the CockroachDB tests to fail which needs fixing.
• [x] The SQLite test for frozen queries is sometimes failing - it's a fragile test, as it depends on timing code.
• [x] We added a proxy function in a few places - we might be able to make this more DRY.