Update dev-requirements dependencies for new versions.
Update dev-requirements dependencies for new versions.

Mercurial now supports python 3 (though hgsubversion does not yet). This
change updates our dev-requirements file to list the appropriate
versions for 2.7 and 3+.

Testing Done:
Installed dev dependencies in various environments and ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/12131/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools