Add support for django.contrib.auth.forms.UsernameField

[ajcerejeira]

django.contrib.auth.forms defines two form fields, UsernameField and ReadOnlyPasswordHashField that are used by django admin website.

At the moment hypothesis does not define a default strategy for UsernameField so this MR adds that support by reusing the same strategy for CharField.

Partial fix for #3417. Incorporates and closes #3420, incorporates and closes #3421 - neither really need a separate release.

[Zac-HD]

Looks like a great start - thanks so much for contributing!

Remaining things: add yourself to the list in AUTHORS.rst, ReadOnlyPasswordHashField support, and adding a hypothesis-python/RELEASE.rst:

RELEASE_TYPE: minor

:func:`~hypothesis.extra.django.from_field` now supports
:class:`~django.contrib.auth.forms.UsernameField` and
:class:`~django.contrib.auth.forms.ReadOnlyPasswordHashField`
from ``django.contrib.auth.forms``.

Thanks to Afonso Silva for reporting and fixing :issue:`3417`.

[ajcerejeira]

Thanks @Zac-HD, I've added myself to the authors list and added a release file.
About ReadOnlyPasswordHashField, this PR already includes support for that, i.e, this field was already working for hypothesis, I simply added a form test to ensure it anyway.

[Zac-HD]

Looks good to me! Once we work out why this broke the docs build (and unbreak it), I'll merge 🎉

And then we can take a look at the other points in #3417 😁

[Zac-HD]

Thanks again @ajcerejeira!