usjon: Add default keyword argument

[antoniobusrod]

Add missing default keyword argument to dump and dumps functions.

Introduced since https://github.com/ultrajson/ultrajson/releases/tag/4.2.0

Relates to ultrajson/ultrajson#470

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Akuli]

The default is actually a function that gets called with one argument to get the default value, so we can use Callable[[Any], Any] instead of just Any.

>>> ujson.encode({'a': object()}, default=(lambda x: "lol"))
'{"a":"lol"}'

We should also add this parameter to dump().

>>> ujson.dump({'a': object()}, sys.stdout, default=(lambda x: "lol"))
{"a":"lol"}>>> 

[Akuli]

It also seems like most of the arguments are keyword-only, even though they aren't marked as keyword-only in the stub:

>>> ujson.dump({'a': object()}, sys.stdout, True, 1, True, True, True, 1, (lambda x: "lol"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: function takes exactly 2 arguments (9 given)

We can fix this in a separate pull request.

[JelleZijlstra]

Could you address @Akuli's feedback?

[AlexWaygood]

Closing due to lack of response from OP