Make ConditionalTypeBinder a context manager.

Now we can use `with binder: X` instead of `binder.push_frame(); X; binder.pop_frame()`

Author: Guido van Rossum

mypy/checker.py

@@ -1,6 +1,7 @@
 """Mypy type checker."""
 
 import itertools
+import contextlib
 
 from typing import (
     Any, Dict, Set, List, cast, Tuple, TypeVar, Union, Optional, NamedTuple
@@ -295,6 +296,12 @@ def push_loop_frame(self):
     def pop_loop_frame(self):
         self.loop_frames.pop()
 
+    def __enter__(self) -> None:
+        self.push_frame()
+
+    def __exit__(self, *args: Any) -> None:
+        self.pop_frame()
+
 
 def meet_frames(*frames: Frame) -> Frame:
     answer = Frame()