This changeset makes it possible to build the Second Life viewer using
Python 3. It is designed to be used with an equivalent Autobuild branch
so that a developer can compile without needing Python 2 on their
machine.

Breaking change: Python 2 support ending

Rather than supporting two versions of Python, including one that was
discontinued at the beginning of the year, this branch focuses on
pouring future effort into Python 3 only. As a result, scripts do not
need to be backwards compatible. This means that build environments,
be they on personal computers and on build agents, need to have a
compatible interpreter.

Notes

- SLVersionChecker will still use Python 2 on macOS
- Fixed the message template url used by template_verifier.py

on the entire current (branch of the) viewer repo before starting any build.

That should be set in TeamCity template hierarchy; don't override it.

result emails

changes for additional packages to be packaged from the same build. see BuildParams for example environment variables used to specify the packages and their channels and sourceids. also updated mac packages to use a larger virtual drive and auto-open on download

added default sourceid and check that sourceid is not blank, hoping to get the windows builds to recognize the value

DRTVWR-324 : Suppress the use of BuildParams to set up the channel and use the TC environment settings instead