Remove contextlib.nullcontext compat shim in favor of pip's version

This removes the need to maintain yet another compatibility shim that is
already handled by pip. pip has a vendored copy of contextlib2, which
contains the same copy of the function.

At some point in the future, pip-tools can use the stdlib version
instead.

piptools/repositories/pypi.py

@@ -20,9 +20,10 @@
 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, contextlib
+from .._compat import PIP_VERSION
 from ..click import progressbar
 from ..exceptions import NoCandidateFound
 from ..logging import log
@@ -407,7 +408,7 @@ def _get_file_hash(self, link):
                     width=32,
                 )
             else:
-                context_manager = contextlib.nullcontext(chunks)
+                context_manager = contextlib2.nullcontext(chunks)
 
             # Iterate over the chosen context manager
             with context_manager as bar: