Historically, inspect.formatargspec() was used to generate Python code

representing function signatures, but modern versions of Python 3.x

provide better support for this through an inspect.Signature object.

This can be populated with information about the function (parameters,

annotations, etc.), and a string can be computed from that.
Internally, inspect.formatargspec() now uses this, but emits a

deprecation warning.
We now use inspect.Signature when available, and feed in parameters

the same way that inspect.formatargspec() does, while avoiding its

deprecation warning. Down the road, we'd be able to use this to support

type annotations as well.

Unit tests pass on Python 2.7, 3.5, 3.6, and 3.7.