Use a different method to lookup base classes on Jython

Jython seems to implement inspect.getmro differently, which causes
any classes with the same name as a class lower in the MRO not to
be returned.

This patch offloads the MRO lookup to a separate function, which
implements different logic for Jython only.

Ref #1024

setuptools/monkey.py

@@ -21,6 +21,19 @@ if you think you need this functionality.
 """
 
 
+def get_mro(cls):
+    """Returns the bases classes for cls sorted by the MRO.
+
+    Works around an issue on Jython where inspect.getmro will not return all
+    base classes if multiple classes share the same name. Instead, this
+    function will return a tuple containing the class itself, and the contents
+    of cls.__bases__ .
+    """
+    if platform.python_implementation() != "Jython":
+        return inspect.getmro(cls)
+    return (cls,) + cls.__bases__
+
+
 def get_unpatched(item):
     lookup = (
         get_unpatched_class if isinstance(item, six.class_types) else
@@ -38,7 +51,7 @@ def get_unpatched_class(cls):
     """
     external_bases = (
         cls
-        for cls in inspect.getmro(cls)
+        for cls in get_mro(cls)
         if not cls.__module__.startswith('setuptools')
     )
     base = next(external_bases)