Fix unformatted f-string in List type error message

[tan7vir]

What

_handle_list in outlines/types/dsl.py raises a TypeError when a List
type is given the wrong number of arguments. The message was built from two
adjacent string literals, and the second one was a plain string rather than an
f-string:

raise TypeError(
    "Only homogeneous lists are supported. You should provide exactly "
    + "one argument to `List`, got {args}."
)

So {args} was printed literally instead of the actual arguments. For example
List[int, str] produced:

Only homogeneous lists are supported. You should provide exactly one argument to `List`, got {args}.

The sibling _handle_dict a few lines down already uses an f-string
(f"Dict must have exactly two type arguments. Got {args}."), so this just
brings _handle_list in line with it.

After

Only homogeneous lists are supported. You should provide exactly one argument to `List`, got (<class 'int'>, <class 'str'>).

Tests

test_dsl_handle_list already exercised this path with a bare
pytest.raises(TypeError), which passed even with the broken message. I
tightened that assertion to match the interpolated args so it fails on the
old string and passes on the fix.

[RobinPicard]

Thanks for the fix!