Twisted is not python 3 compatible. So for now create a python2
venv and install twisted in it when running provision.py in python3
mode and use twisted from the python2 venv.
tools/provision.py: Create directory var/uploads.
zproject/local_settings_template.py: Update Upload dir to var/uploads.
zproject/dev_settings.py: Update upload dir to var/uploads.
The purpose of this is to move a lot of the log and other generated
files used by the Zulip development environment into a consistent
hierarchy.
We also need to create this in tools/build-release-tarball as well,
since that runs a development environment out of a temporary
directory.
When provision.py is run using python 2, retain original behavior
of creating a python 2 venv using requirements/py2_dev.txt and
creating a python 3 venv using requirements/mypy.txt.
When provision.py is run using python 3, install a single python 3
venv with requirements/py3_dev.txt.