Use built in getfullargspec if __signature__ is present

Repaired the test suite to work with the 5.x series of the ``decorator``
module, which now appears to make use of the ``__signature__`` attribute.

Change-Id: Id0ef2242ed7e400f5b2445df4be44b34598ae1e8

Author: zzzeek

docs/build/unreleased/dec.rst

@@ -0,0 +1,5 @@
+.. change::
+    :tags: bug, regression, tests
+
+    Repaired the test suite to work with the 5.x series of the ``decorator``
+    module, which now appears to make use of the ``__signature__`` attribute.

dogpile/util/compat.py

@@ -28,6 +28,11 @@ def inspect_getfullargspec(func):
 
     """
 
+    # if a Signature is already present, as is the case with newer
+    # "decorator" package, defer back to built in
+    if hasattr(func, "__signature__"):
+        return inspect.getfullargspec(func)
+
     if inspect.ismethod(func):
         func = func.__func__
     if not inspect.isfunction(func):