stub for statistics (#1021)

* stub for statistics

* fix missing imports

* drop int from the typevar

Author: JelleZijlstra

stdlib/3.4/statistics.pyi

@@ -0,0 +1,24 @@
+# Stubs for statistics
+
+from decimal import Decimal
+from fractions import Fraction
+import sys
+from typing import Iterable, Optional, TypeVar
+
+# Most functions in this module accept homogeneous collections of one of these types
+_Number = TypeVar('_Number', float, Decimal, Fraction)
+
+class StatisticsError(ValueError): ...
+
+def mean(data: Iterable[_Number]) -> _Number: ...
+if sys.version_info >= (3, 6):
+    def harmonic_mean(data: Iterable[_Number]) -> _Number: ...
+def median(data: Iterable[_Number]) -> _Number: ...
+def median_low(data: Iterable[_Number]) -> _Number: ...
+def median_high(data: Iterable[_Number]) -> _Number: ...
+def median_grouped(data: Iterable[_Number]) -> _Number: ...
+def mode(data: Iterable[_Number]) -> _Number: ...
+def pstdev(data: Iterable[_Number], mu: Optional[_Number] = ...) -> _Number: ...
+def pvariance(data: Iterable[_Number], mu: Optional[_Number] = ...) -> _Number: ...
+def stdev(data: Iterable[_Number], xbar: Optional[_Number] = ...) -> _Number: ...
+def variance(data: Iterable[_Number], xbar: Optional[_Number] = ...) -> _Number: ...