ForeignKey static typing

[dantownsend]

Resolves #680

Related to #436

ForeignKey static typing in joins

The 'holy grail' for Piccolo for a long time has been to get 'perfect' static typing for ForeignKey columns.

If there was some kind of Proxy type in the typing module, it would be easy, but there isn't. Here's a related issue (python/typing#802) for anyone who wants to track the progress.

I've come up with a solution. This query will continue to work:

await Band.select(Band.name, Band.manager.name)

But this PR allows you to do this:

await Band.select(Band.name, Band.manager._.name)

Note the addition of the _ after the foreign key. It then has perfect static typing.

To remember the syntax, ._. looks a bit like a connector in a diagram.

When Python adds a Proxy type to the typing module, we can make Band.manager.name behave the same as Band.manager._.name, but for now I think it's the best solution.

With perfect static typing of foreign keys, it is really cool, because Mypy and Pylance will detect any column name typos when doing joins.

get_related

This PR also improves the typing for get_related. For example:

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

# Oops, I need the manager object too:
manager = await band.get_related(Band.manager)

In the above example, the type for manager is Manager | None.

Remaining tasks

• Fix some minor linter errors
• Add some tests to make sure the new syntax works
• Update the docs

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (4fb169d) 87.91% compared to head (92fb265) 91.99%.

Files	Patch %	Lines
piccolo/columns/column_types.py	82.35%	3 Missing ⚠️
piccolo/table.py	71.42%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #914      +/-   ##
==========================================
+ Coverage   87.91%   91.99%   +4.07%     
==========================================
  Files         107      107              
  Lines        8145     8165      +20     
==========================================
+ Hits         7161     7511     +350     
+ Misses        984      654     -330     

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