NOTICKET: Add paul-mclendahand config to bedrock

https://github.com/willkg/paul-mclendahand makes maintenance-PR handling so much easier than one-by-one

This changeset adds a default config to make it pretty much instant to use after installation.

Note that this addition doesn't specify which remote to use, so we may need to tune this if 'origin' means different things to different contributors.

setup.cfg

@@ -22,3 +22,9 @@ testpaths =
     bedrock
     lib
     tests
+
+[tool:paul-mclendahand]
+# Config for use with https://github.com/willkg/paul-mclendahand
+github_user=mozilla
+github_project=bedrock
+main_branch=master