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/crypt.pyi

@@ -0,0 +1,22 @@
+import sys
+from typing import List, Optional, Union
+
+if sys.version_info >= (3, 3):
+    class _Method: ...
+    METHOD_CRYPT: _Method
+    METHOD_MD5: _Method
+    METHOD_SHA256: _Method
+    METHOD_SHA512: _Method
+    if sys.version_info >= (3, 7):
+        METHOD_BLOWFISH: _Method
+
+    methods: List[_Method]
+
+    if sys.version_info >= (3, 7):
+        def mksalt(method: Optional[_Method] = ..., *, rounds: Optional[int] = ...) -> str: ...
+    else:
+        def mksalt(method: Optional[_Method] = ...) -> str: ...
+    def crypt(word: str, salt: Optional[Union[str, _Method]] = ...) -> str: ...
+
+else:
+    def crypt(word: str, salt: str) -> str: ...