Revert removed piptools._compat.contextlib (#1338)

This reverts commit 0036708.

piptools/_compat/contextlib.py

@@ -0,0 +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: Optional[_T] = None) -> None:
+        self.enter_result = enter_result
+
+    def __enter__(self) -> Optional[_T]:
+        return self.enter_result
+
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
+    ) -> Optional[bool]:
+        pass

piptools/repositories/pypi.py

@@ -21,10 +21,9 @@
 from pip._internal.utils.misc import normalize_path
 from pip._internal.utils.temp_dir import TempDirectory, global_tempdir_manager
 from pip._internal.utils.urls import path_to_url, url_to_path
-from pip._vendor import contextlib2
 from pip._vendor.requests import RequestException
 
-from .._compat import PIP_VERSION
+from .._compat import PIP_VERSION, contextlib
 from ..exceptions import NoCandidateFound
 from ..logging import log
 from ..utils import (
@@ -407,7 +406,7 @@ def _get_file_hash(self, link):
                     width=32,
                 )
             else:
-                context_manager = contextlib2.nullcontext(chunks)
+                context_manager = contextlib.nullcontext(chunks)
 
             # Iterate over the chosen context manager
             with context_manager as bar: