I'd like to request an exception that allows the mercurial package to use Python 2 during buildtime and runtime. There are no other dependencies needed beside the Python interpreter.

Rationale:

1. Upstream mercurial is Python 3 ready, but there are many "plugins" in the mercurial ecosystem that are not ready for the switch. ideally, we should have had both python2- and python3-based mercurails in Fedora 31 to test the transition, but unfortunately, work there was nit yet completed. The plan is to do this in Fedora 32 and eventually get rid of python2-based mercurial once the ecosystem is ready.

2. There are many non-Python packages that (Build)Require mercurail in Fedora and removing all of them would be too intrusive.

3. Mercurial has no other dependencies, so keeping it running on Python 2 does block maintainers of any other Python 2 packages.

Notes:

1. There are some Python packages that use mercurial, but this exception request is leaving them intentionally out. Work remains to be done to determine whether they need to stay on Python 2 or not. If they do, separate exceptions requests might follow.

2. Currently, mercurial uses python2-docutils to build the docs. We are not requesting an exception for python2-docutils, either the docs will be generated by python3-docutils or pregenerated.

Context: https://bugzilla.redhat.com/show_bug.cgi?id=1737931