Add render_kw support (#40)

[ghost]

WTForms choices iterators have recently changed to expect a tuple of
4 elements. This change provides a callable field get_render_kw which
is similar to get_pk and get_label.

Note: blank form values have no obj reference, so it doesn't make
sense to call get_render_kw in that scope. For now an empty dict
is returned, but maybe it should provide a blank_render_kw
callable? That's a bit of a design decision that's not up to me.

I've had a quick look at fixing up the unit tests to make them pass, but
nothing substantial has been added to test the get_render_kw return
values.

[azmeuk]

LGTM. Can you add a changelog entry and fix the style CI job please?

Note: blank form values have no obj reference, so it doesn't make
sense to call get_render_kw in that scope. For now an empty dict
is returned, but maybe it should provide a blank_render_kw
callable? That's a bit of a design decision that's not up to me.

Let's do it simple for now, and wait to see if someone ever need a more complexe feature.

[ghost]

Style fixed. Thanks again.

[ghost]

@jace All good from me, just waiting on someone to merge it.

[jace]

Folks, there's no PyPI release yet. Why was that comment deleted? It affects everyone using wtforms-sqlalchemy with the latest release of wtforms.

[ghost]

@jace Not sure. I'm not associated with the maintainers, sorry.

[jace]

I just noticed WTForms 3.1.1 has been released, making render_kw optional (in pallets-eco/wtforms#816). The existing release on PyPI should continue to work. I've opened #45 to remove the pin on sqlalchemy<2.

[azmeuk]

I was waiting on conflicts on #44 being addressed before cutting a new release.
wtforms 3.1.1 restores the compatibility, but with a deprecation warning.