Remove PIP_NO_PYTHON_VERSION_WARNING.

Ref pypa/pip#13154

Author: jaraco

.github/workflows/main.yml

@@ -21,7 +21,6 @@ env:
 
   # Suppress noisy pip warnings
   PIP_DISABLE_PIP_VERSION_CHECK: 'true'
-  PIP_NO_PYTHON_VERSION_WARNING: 'true'
   PIP_NO_WARN_SCRIPT_LOCATION: 'true'
 
   # Ensure tests can sense settings about the environment