This patch changes it so that `inspect.signature` is used instead of
`inspect.getfullargspec` to get a function's signature, when using Python 3.

Python 3.3 introduced the `inspect.signature` function as a new way to get the
signature of a function (as an alternative to `inspect.getargspec` and
`inspect.getfullargspec`).  `inspect.signature` has the advantage that it
preserves the signature of a decorated function if `functools.wraps` is used to
decorated the wrapper function.  Having a function's signature available is very
hepful, especially when testing things out in a REPL.

Instead of referencing parameter kinds as class attributes on the private
`_ParameterKind` class we reference them on the public `Parameter` class.  This
has two advantages:

1) We don't use a private interface.
2) The class attributes on `_ParameterKind` have only been added in Python 3.5,
   but on `Parameter` they have existed since Python 3.3.

Some built-in functions (e.g. `map`) can't be inspected with
`inspect.getargspec`, `inspect.getfullargspec` or `inspect.signature`.  The
exceptions from `inspect.getargspec` and `inspect.getfullargspec` are all caught
in the code, but `inspect.signature` raises a `ValueError` instead of a
`TypeError`.  This exception is now also caught.