add bytearray.copy to Python 3 (#2133)

Fixes #1715.

2.7's bytearray doesn't have it.

Author: JelleZijlstra

stdlib/3/builtins.pyi

@@ -429,6 +429,7 @@ class bytearray(MutableSequence[int], ByteString):
         def count(self, sub: Union[bytes, int], start: Optional[int] = ..., end: Optional[int] = ...) -> int: ...
     else:
         def count(self, sub: bytes, start: Optional[int] = ..., end: Optional[int] = ...) -> int: ...
+    def copy(self) -> bytearray: ...
     def decode(self, encoding: str = ..., errors: str = ...) -> str: ...
     def endswith(self, suffix: bytes) -> bool: ...
     def expandtabs(self, tabsize: int = ...) -> bytearray: ...