Piccolo

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Fixed a bug with ``ModelBuilder`` and nullable columns (see `PR 462 <https://github.com/piccolo-orm/piccolo/pull/462>`_).
Thanks to fiolet069 for reporting this issue.

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The ``ModelBuilder`` class, which is used to generate mock data in tests, now
supports ``Array`` columns. Courtesy backwardspy.

Lots of internal code optimisations and clean up. Courtesy yezz123.

Added docs for troubleshooting common MyPy errors.

Also thanks to adriangb for helping us with our dependency issues.

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Fixed a bug with auto migrations. If renaming multiple columns at once, it
could get confused. Thanks to theelderbeever for reporting this issue, and
sinisaos for helping to replicate it. See `PR 457 <https://github.com/piccolo-orm/piccolo/pull/457>`_.

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We ran a profiler on the Piccolo codebase and identified some optimisations.
For example, we were calling ``self.querystring`` multiple times in a method,
rather than assigning it to a local variable.

We also ran a linter which identified when list / set / dict comprehensions
could be more efficient.

The performance is now slightly improved (especially when fetching large
numbers of rows from the database).

Example query times on a MacBook, when fetching 1000 rows from a local Postgres
database (using ``await SomeTable.select()``):

* 8 ms without a connection pool
* 2 ms with a connection pool

As you can see, having a connection pool is the main thing you can do to
improve performance.

Thanks to AliSayyah for all his work on this.

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Made improvements to ``piccolo schema generate``, which automatically generates
Piccolo ``Table`` classes from an existing database.

There were situations where it would fail ungracefully when it couldn't parse
an index definition. It no longer crashes, and we print out the problematic
index definitions. See `PR 449 <https://github.com/piccolo-orm/piccolo/pull/449>`_.
Thanks to gmos for originally reporting this issue.

We also improved the error messages if schema generation fails for some reason
by letting the user know which table caused the error. Courtesy AliSayyah.

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We used to raise a ``ValueError`` if a column was both ``null=False`` and
``default=None``. This has now been removed, as there are situations where
it's valid for columns to be configured that way. Thanks to gmos for
suggesting this change.

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