pull requests: prevent adding DEFAULT user as reviewer Add a helper method to resolve reviewers, with an added check to prevent the adding of the DEFAULT user as reviewer. The __add_reviewers method, although internal, had a troubling interface where the method was responsible for resolving reviewers, but the caller was responsible for resolving mention_recipients. The method is changed to always take two sets of database User objects, leaving the resolution to the caller in both cases.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
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