tests: git changeset authors need to have the format 'username <user@example.com>' New verions of dulwich caused tests like test_compare_forks_on_branch_extra_commits_git to fail because of this. Since this is fixed now, re-allow dulwich versions 0.18.6 and newer.

# 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
.