This PR was created by Knope. Merging it will create a new release

### Features

#### Export `Unset` types from generated `types.py` (#927)

#### Generate properties for some boolean enums

If a schema has both `type = "boolean"` and `enum` defined, a normal
boolean property will now be created.
Previously, the generator would error. 

Note that the generate code _will not_ correctly limit the values to the
enum values. To work around this, use the
OpenAPI 3.1 `const` instead of `enum` to generate Python `Literal`
types.

Thanks for reporting #922 @macmoritz!

### Fixes

#### Do not stop generation for invalid enum values

This generator only supports `enum` values that are strings or integers.
Previously, this was handled at the parsing level, which would cause the
generator to fail if there were any unsupported values in the document.
Now, the generator will correctly keep going, skipping only endpoints
which contained unsupported values.

Thanks for reporting #922 @macmoritz!

#### Fix lists within unions

Fixes #756 and #928. Arrays within unions (which, as of 0.17 includes
nullable arrays) would generate invalid code.

Thanks @kgutwin and @diesieben07!

#### Simplify type checks for non-required unions

Co-authored-by: GitHub <[email protected]>