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

SL-13705 - added --deleted option to collect info on no-longer-needed text in translated files. Processing needs to be automated as the list is unfeasibly long.

SL-13705 - language list defaults to the standard supported set of FR, ES, IT, PT, JA, DE. '--lang all' will also use this list

SL-13705 - lock all columns except C in the excel file (apparently google sheets does not respect this setting on import)