Fix pylint E0015: Unrecognized option found: bad-functions

bad-functions is an option recognized by the bad-builtin
checker, which used to be enabled by default but is now
an optional plugin:
https://pylint.pycqa.org/en/v2.15.2/user_guide/checkers/extensions.html#deprecated-builtins-checker-documentation

.pylintrc

+[MAIN]
+load-plugins=pylint.extensions.bad_builtin
+
 [MASTER]
 extension-pkg-whitelist=gevent.greenlet,gevent.libuv._corecffi,gevent.libev._corecffi,gevent.libev._corecffi.lib,gevent.local,gevent._ident