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/zlib.pyi

@@ -0,0 +1,66 @@
+import sys
+from array import array
+from typing import Any, Optional, Union
+
+DEFLATED: int
+DEF_MEM_LEVEL: int
+MAX_WBITS: int
+ZLIB_VERSION: str
+Z_BEST_COMPRESSION: int
+Z_BEST_SPEED: int
+Z_DEFAULT_COMPRESSION: int
+Z_DEFAULT_STRATEGY: int
+Z_FILTERED: int
+Z_FINISH: int
+Z_FIXED: int
+Z_FULL_FLUSH: int
+Z_HUFFMAN_ONLY: int
+Z_NO_FLUSH: int
+Z_RLE: int
+Z_SYNC_FLUSH: int
+if sys.version_info >= (3,):
+    DEF_BUF_SIZE: int
+    ZLIB_RUNTIME_VERSION: str
+
+class error(Exception): ...
+
+class _Compress:
+    def compress(self, data: bytes) -> bytes: ...
+    def flush(self, mode: int = ...) -> bytes: ...
+    def copy(self) -> _Compress: ...
+
+class _Decompress:
+    unused_data: bytes
+    unconsumed_tail: bytes
+    if sys.version_info >= (3,):
+        eof: bool
+    def decompress(self, data: bytes, max_length: int = ...) -> bytes: ...
+    def flush(self, length: int = ...) -> bytes: ...
+    def copy(self) -> _Decompress: ...
+
+def adler32(__data: bytes, __value: int = ...) -> int: ...
+def compress(__data: bytes, level: int = ...) -> bytes: ...
+
+if sys.version_info >= (3,):
+    def compressobj(
+        level: int = ...,
+        method: int = ...,
+        wbits: int = ...,
+        memLevel: int = ...,
+        strategy: int = ...,
+        zdict: Optional[bytes] = ...,
+    ) -> _Compress: ...
+
+else:
+    def compressobj(
+        level: int = ..., method: int = ..., wbits: int = ..., memlevel: int = ..., strategy: int = ...
+    ) -> _Compress: ...
+
+def crc32(__data: Union[array[Any], bytes], __value: int = ...) -> int: ...
+def decompress(__data: bytes, wbits: int = ..., bufsize: int = ...) -> bytes: ...
+
+if sys.version_info >= (3,):
+    def decompressobj(wbits: int = ..., zdict: bytes = ...) -> _Decompress: ...
+
+else:
+    def decompressobj(wbits: int = ...) -> _Decompress: ...