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: Patch coverage is 88.46154% with 15 lines in your changes missing coverage. Please review.

Project coverage is 88.41%. Comparing base (70dac99) to head (6d8bc36).
Report is 3 commits behind head on master.

Files	Patch %	Lines
piccolo/table.py	54.54%	5 Missing ⚠️
piccolo/query/methods/select.py	66.66%	3 Missing ⚠️
piccolo/engine/cockroach.py	0.00%	2 Missing ⚠️
piccolo/engine/postgres.py	88.23%	2 Missing ⚠️
piccolo/engine/sqlite.py	84.61%	2 Missing ⚠️
piccolo/utils/encoding.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
- Coverage   89.27%   88.41%   -0.86%     
==========================================
  Files         114      114              
  Lines        8333     8383      +50     
==========================================
- Hits         7439     7412      -27     
- Misses        894      971      +77     

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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).