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/cmath.pyi

@@ -0,0 +1,42 @@
+import sys
+from typing import SupportsComplex, SupportsFloat, Tuple, Union
+
+e: float
+pi: float
+if sys.version_info >= (3, 6):
+    inf: float
+    infj: complex
+    nan: float
+    nanj: complex
+    tau: float
+
+_C = Union[SupportsFloat, SupportsComplex, complex]
+
+def acos(__z: _C) -> complex: ...
+def acosh(__z: _C) -> complex: ...
+def asin(__z: _C) -> complex: ...
+def asinh(__z: _C) -> complex: ...
+def atan(__z: _C) -> complex: ...
+def atanh(__z: _C) -> complex: ...
+def cos(__z: _C) -> complex: ...
+def cosh(__z: _C) -> complex: ...
+def exp(__z: _C) -> complex: ...
+
+if sys.version_info >= (3, 5):
+    def isclose(a: _C, b: _C, *, rel_tol: SupportsFloat = ..., abs_tol: SupportsFloat = ...) -> bool: ...
+
+def isinf(__z: _C) -> bool: ...
+def isnan(__z: _C) -> bool: ...
+def log(__x: _C, __y_obj: _C = ...) -> complex: ...
+def log10(__z: _C) -> complex: ...
+def phase(__z: _C) -> float: ...
+def polar(__z: _C) -> Tuple[float, float]: ...
+def rect(__r: float, __phi: float) -> complex: ...
+def sin(__z: _C) -> complex: ...
+def sinh(__z: _C) -> complex: ...
+def sqrt(__z: _C) -> complex: ...
+def tan(__z: _C) -> complex: ...
+def tanh(__z: _C) -> complex: ...
+
+if sys.version_info >= (3,):
+    def isfinite(__z: _C) -> bool: ...