Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/audioop.pyi

@@ -0,0 +1,42 @@
+from typing import Optional, Tuple
+
+AdpcmState = Tuple[int, int]
+RatecvState = Tuple[int, Tuple[Tuple[int, int], ...]]
+
+class error(Exception): ...
+
+def add(__fragment1: bytes, __fragment2: bytes, __width: int) -> bytes: ...
+def adpcm2lin(__fragment: bytes, __width: int, __state: Optional[AdpcmState]) -> Tuple[bytes, AdpcmState]: ...
+def alaw2lin(__fragment: bytes, __width: int) -> bytes: ...
+def avg(__fragment: bytes, __width: int) -> int: ...
+def avgpp(__fragment: bytes, __width: int) -> int: ...
+def bias(__fragment: bytes, __width: int, __bias: int) -> bytes: ...
+def byteswap(__fragment: bytes, __width: int) -> bytes: ...
+def cross(__fragment: bytes, __width: int) -> int: ...
+def findfactor(__fragment: bytes, __reference: bytes) -> float: ...
+def findfit(__fragment: bytes, __reference: bytes) -> Tuple[int, float]: ...
+def findmax(__fragment: bytes, __length: int) -> int: ...
+def getsample(__fragment: bytes, __width: int, __index: int) -> int: ...
+def lin2adpcm(__fragment: bytes, __width: int, __state: Optional[AdpcmState]) -> Tuple[bytes, AdpcmState]: ...
+def lin2alaw(__fragment: bytes, __width: int) -> bytes: ...
+def lin2lin(__fragment: bytes, __width: int, __newwidth: int) -> bytes: ...
+def lin2ulaw(__fragment: bytes, __width: int) -> bytes: ...
+def max(__fragment: bytes, __width: int) -> int: ...
+def maxpp(__fragment: bytes, __width: int) -> int: ...
+def minmax(__fragment: bytes, __width: int) -> Tuple[int, int]: ...
+def mul(__fragment: bytes, __width: int, __factor: float) -> bytes: ...
+def ratecv(
+    __fragment: bytes,
+    __width: int,
+    __nchannels: int,
+    __inrate: int,
+    __outrate: int,
+    __state: Optional[RatecvState],
+    __weightA: int = ...,
+    __weightB: int = ...,
+) -> Tuple[bytes, RatecvState]: ...
+def reverse(__fragment: bytes, __width: int) -> bytes: ...
+def rms(__fragment: bytes, __width: int) -> int: ...
+def tomono(__fragment: bytes, __width: int, __lfactor: float, __rfactor: float) -> bytes: ...
+def tostereo(__fragment: bytes, __width: int, __lfactor: float, __rfactor: float) -> bytes: ...
+def ulaw2lin(__fragment: bytes, __width: int) -> bytes: ...