Install Tempita from a fork due to `use_2to3` deprecation

Fix https://github.com/galaxyproject/galaxy/issues/12412 .

We have a dependency on `Tempita` (through `sqlalchemy-migrate`), which is
abandonware and uses the `use_2to3=True` option of `setup()` in its
`setup.py` .
The new Setuptools v58.0.0 removed support for 2to3 during builds:
https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v5800
As a result Tempita is installed at even older version and importing it
raises a SyntaxError.

For Galaxy as a full application this has been solved by creating a wheel on
https://wheels.galaxyproject.org . But the `galaxy-data` package (which also
depends on `sqlalchemy-migrate`) cannot automatically install the
tempita-0.5.2 wheel from our alternative package index when installed via
`pip install galaxy-data` as there is no syntax to specify that.

I've therefore resurrected Tempita from the Bitbucket Mercurial repo (thanks
to Software Heritage backups!
https://bitbucket-archive.softwareheritage.org/projects/ia/ianb/tempita.html
), converted to a GitHub repo and updated its code to Python 3.6 with 2to3
and pyupgrade.