Move Interrupted exception class out of Session

_pytest/main.py

@@ -502,11 +502,13 @@ class Item(Node):
 class NoMatch(Exception):
     """ raised if matching cannot locate a matching names. """
 
-class Session(FSCollector):
-    class Interrupted(KeyboardInterrupt):
+class Interrupted(KeyboardInterrupt):
     """ signals an interrupted test run. """
     __module__ = 'builtins' # for py3
 
+class Session(FSCollector):
+    Interrupted = Interrupted
+
     def __init__(self, config):
         FSCollector.__init__(self, config.rootdir, parent=None,
                              config=config, session=self)