Kallithea port to Python 3

[Mads Kiilerich]

Hi

After heavy christmas crunch mode, https://kallithea-scm.org/repos/ is 
now running on Python 3. It generally works, and the test suite pass.

The port is using more than 100 changes that haven't landed yet. The 
changes can be seen on 
https://kallithea-scm.org/repos/kallithea-incoming/changelog/py3/ .

The changes have mostly been structured as clean changes, mainly with 
cleanup and refactorings, and it can land safely on "default" branch 
while still using py2. Then we will move on and land changes to switch 
the "default" branch to py3. The next major release will probably be py3 
only. We will not try to support 2 and 3 simultaneously with the same 
codebase. While finishing up py3, minor releases for py2 will be made 
from the stable branch.

I'm sure the changes will introduce regressions. Testing and review 
would thus be much appreciated. You can get the changes with "hg pull 
https://kallithea-scm.org/repos/kallithea-incoming -r py3". Feedback can 
be given with mail here to this list - perhaps referencing follow-up 
changes hosted on your preferred hosting site.

/Mads