Improve contextlib (#406)

* remove old, new stubgen

* comment everything

* contextlib done

* use TypeVar instead of overload

* py2 done

Author: tharvik

stdlib/2.7/contextlib.pyi

@@ -0,0 +1,53 @@
+# Stubs for contextlib
+
+from typing import (
+    Any, Callable, Generator, IO, Optional, Type,
+    Generic, TypeVar,
+)
+from types import TracebackType
+import sys
+
+_T = TypeVar('_T')
+_ExitFunc = Callable[[Optional[Type[BaseException]],
+                      Optional[Exception],
+                      Optional[TracebackType]], bool]
+_CM_EF = TypeVar('_CM_EF', ContextManager, _ExitFunc)
+
+# TODO already in PEP, have to get added to mypy
+class ContextManager(Generic[_T]):
+    def __enter__(self) -> _T: ...
+    def __exit__(self, exc_type: Optional[Type[BaseException]],
+                 exc_val: Optional[Exception],
+                 exc_tb: Optional[TracebackType]) -> bool: ...
+
+def contextmanager(func: Callable[..., Generator[None, None, None]]) -> Callable[..., ContextManager[None]]: ...
+
+if sys.version_info < (3,):
+    def nested(*mgr: ContextManager[Any]) -> ContextManager[None]: ...
+
+class closing(Generic[_T], ContextManager[_T]):
+    def __init__(self, thing: _T) -> None: ...
+
+if sys.version_info >= (3, 4):
+    class suppress(ContextManager[None]):
+        def __init__(self, *exceptions: Type[BaseException]) -> None: ...
+
+    class redirect_stdout(ContextManager[None]):
+        def __init__(self, new_target: IO[str]) -> None: ...
+
+if sys.version_info >= (3, 5):
+    class redirect_stderr(ContextManager[None]):
+        def __init__(self, new_target: IO[str]) -> None: ...
+
+if sys.version_info >= (3,):
+    class ContextDecorator:
+        def __call__(self, func: Callable[..., None]) -> Callable[..., ContextManager[None]]: ...
+
+    class ExitStack(ContextManager[ExitStack]):
+        def __init__(self) -> None: ...
+        def enter_context(self, cm: ContextManager[_T]) -> _T: ...
+        def push(self, exit: _CM_EF) -> _CM_EF: ...
+        def callback(self, callback: Callable[..., None],
+                     *args: Any, **kwds: Any) -> Callable[..., None]: ...
+        def pop_all(self) -> ExitStack: ...
+        def close(self) -> None: ...