* Add support for generic optional parameters
* Tests for passing non-optional arguments to optional parameters
* Remove now-redundant is_class_value handling
This parameter has since been removed from infer_type_vars methods,
much simplifying the code.
Unfortunately I can't recall exactly what it was that this test
case was trying to validate, however on a second look it turns
out that it was working by accident and did not represent a valid
use of generic type vars in Python (which cannot be used completely
unbound as this was).
While these definitely _ought_ to work on Python 2.7, the annotation
support there is very limited and as Python 2 is deprecated it
doesn't seem worth it.
