Added a get_venv_dependencies() function in setup_venv.py which
returns VENV_DEPENDENCIES according to the vendor and os_version.
The reason for adding this function was because python-dev will be
depreciated in Focal but can be used as python2-dev so when adding
support for Focal VENV_DEPENDENCIES should to be os_version dependent.
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.