Fix return type of __exit__ to bool

Author: deveshks

Diff for: src/pip/_internal/build_env.py

@@ -132,7 +132,7 @@ def __exit__(
         exc_val,  # type: Optional[BaseException]
         exc_tb  # type: Optional[TracebackType]
     ):
-        # type: (...) -> None
+        # type: (...) -> bool
         for varname, old_value in self._save_env.items():
             if old_value is None:
                 os.environ.pop(varname, None)
@@ -222,7 +222,7 @@ def __exit__(
         exc_val,  # type: Optional[BaseException]
         exc_tb  # type: Optional[TracebackType]
     ):
-        # type: (...) -> None
+        # type: (...) -> bool
         pass
 
     def cleanup(self):