Extend deprecation period by 1 release cycle

Author: sbidoul

src/pip/_internal/commands/install.py

@@ -384,7 +384,7 @@ def run(self, options, args):
                         )
                     ),
                     replacement="to fix the wheel build issue reported above",
-                    gone_in="20.3",
+                    gone_in="21.0",
                     issue=8368,
                 )