Python 3.8 added syntax for positional-only arguments, which is not

compatible with prior releases. This involved changes to the CodeType

parameters and function signatures.
It unfortunately did not implement this new support in

inspect.getfullargspec(), meaning we finally needed to move fully over

to the inspect.Signature support. This involved a fair amount of work

in the inspection and formatting code for functions. It also required

that we call into an internal function in the inspect module, which is

needed in order to retain some state in the signature that is otherwise

not available publicly.
The end result is that we can now spy on functions with positional-only

arguments in Python 3.8+.

Unit tests pass on Python 2.7 and 3.5-3.8.
Ran the full Djblets and Review Board test suites on all supported

versions of Python without errors.