As FunctionSpy got more complicated and continued to add compatibility

with newer versions of Python, the code got increasingly complex and

littered with Python version checks. This could make it more challenging

to reason about some parts of code.
To address this, this change introduces a new set of FunctionSig

classes, which consist of BaseFunctionSig, FunctionSigPy2, and

FunctionSigPy3.
These are responsible for taking a function and storing information on

the function type, owner, and arguments. It's also responsible for

generating code for a function's argument signature or for referencing a

variable defined in a spy-generated function, for the purposes of

generating those functions.
The Py2 and Py3 versions of these classes implement the logic for

Python 2 and Python 3, respectively. There's some common logic in the

base class, but the bulk are in these. They also contain constants used

for function attribute states, which used to be defined per-version in

spies.py.
A FunctionSig alias exists that points to the appropriate class for

the current version of Python.
FunctionSpy generates an instance of this for the provided function

up-front, and uses it to figure out the owner and other information. It

ditches the argspec-related methods in favor of calls to this. It also

provides some property wrappers for attributes on the signature, for

compatibility purposes.
Version checks are still required in FunctionSpy for many purposes,

such as when filling out attributes for a CodeType or FunctionType,

but a lot of the complexities are now isolated to the FunctionSigs.
This should help keep the codebase more maintainable and future-proof as

we go on.

Unit tests pass on Python 2.7 and 3.5-3.8.
Ran Djblets and Review Board unit test suites with these Python

versions to make sure nothing broke in real-world usage.