FIX: Remove deprecation warning from vendored pkg_resources

Leave a comment to flag the change for the next time setuptools is
vendored.

Fixes pypagh-12243.

src/pip/_vendor/pkg_resources/__init__.py

@@ -101,12 +101,10 @@
 _namespace_packages = None
 
 
-warnings.warn(
-    "pkg_resources is deprecated as an API. "
-    "See https://setuptools.pypa.io/en/latest/pkg_resources.html",
-    DeprecationWarning,
-    stacklevel=2,
-)
+# Patch: Remove deprecation warning from vendored pkg_resources.
+# Setting PYTHONWARNINGS=error to verify builds produce no warnings
+# causes immediate exceptions.
+# See https://github.com/pypa/pip/issues/12243
 
 
 _PEP440_FALLBACK = re.compile(r"^v?(?P<safe>(?:[0-9]+!)?[0-9]+(?:\.[0-9]+)*)", re.I)