Add __debuggerskip__ as special local

__debuggerskip__ is a special variable used by IPython, similar to
__tracebackhide__.

pyflakes/checker.py

@@ -548,7 +548,7 @@ class FunctionScope(Scope):
     """
     usesLocals = False
     alwaysUsed = {'__tracebackhide__', '__traceback_info__',
-                  '__traceback_supplement__'}
+                  '__traceback_supplement__', '__debuggerskip__'}
 
     def __init__(self):
         super().__init__()

pyflakes/test/test_other.py

@@ -1349,6 +1349,16 @@ def helper():
                 __tracebackhide__ = True
         """)
 
+    def test_debuggerskipSpecialVariable(self):
+        """
+        Do not warn about unused local variable __debuggerskip__, which is
+        a special variable for IPython.
+        """
+        self.flakes("""
+            def helper():
+                __debuggerskip__ = True
+        """)
+
     def test_ifexp(self):
         """
         Test C{foo if bar else baz} statements.