Add math.tau to typeshed (#801)

As per https://www.python.org/dev/peps/pep-0628/

Author: afrieder

stdlib/2and3/math.pyi

@@ -10,6 +10,8 @@ pi = ...  # type: float
 if sys.version_info >= (3, 5):
     inf = ...  # type: float
     nan = ...  # type: float
+if sys.version_info >= (3, 6):
+    tau = ...  # type: float
 
 def acos(x: float) -> float: ...
 def acosh(x: float) -> float: ...