Add type annotations for nullcontext()

piptools/_compat/contextlib.py

@@ -1,18 +1,31 @@
 # Ported from python 3.7 contextlib.py
+from types import TracebackType
+from typing import Optional, Type, TypeVar
+
+_T = TypeVar("_T")
+
+
 class nullcontext:
     """Context manager that does no additional processing.
     Used as a stand-in for a normal context manager, when a particular
     block of code is only sometimes used with a normal context manager:
     cm = optional_cm if condition else nullcontext()
     with cm:
         # Perform operation, using optional_cm if condition is True
+
+    TODO: replace with `contextlib.nullcontext()` after Python 3.6 being dropped
     """
 
-    def __init__(self, enter_result=None):
+    def __init__(self, enter_result: Optional[_T] = None) -> None:
         self.enter_result = enter_result
 
-    def __enter__(self):
+    def __enter__(self) -> Optional[_T]:
         return self.enter_result
 
-    def __exit__(self, *excinfo):
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
+    ) -> Optional[bool]:
         pass