Pylance fixes

[dantownsend]

Builds on the changes in PR #905

Similarly to Mypy, Pylance has become much stricter / more accurate over time. There are now quite a few minor type errors being reported by Pylance.

As VSCode is such a popular code editor it would be good to clean up most (if not all) of these minor errors.

I'm not sure how easy it is to add Pyright (which powers Pylance) as a linter to our CI, but will investigate.

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (b8026da) 92.03% compared to head (c05bb5d) 78.61%.

Files	Patch %	Lines
piccolo/engine/postgres.py	33.33%	6 Missing ⚠️
piccolo/table.py	0.00%	4 Missing ⚠️
piccolo/columns/defaults/base.py	0.00%	3 Missing ⚠️
piccolo/apps/playground/commands/run.py	87.50%	1 Missing ⚠️
piccolo/utils/encoding.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #906       +/-   ##
===========================================
- Coverage   92.03%   78.61%   -13.42%     
===========================================
  Files         107      107               
  Lines        8145     8165       +20     
===========================================
- Hits         7496     6419     -1077     
- Misses        649     1746     +1097     

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[dantownsend]

This PR is insanely big - I'm going to break it down into smaller chunks, and merge those in (starting with any changes to test files).