On Mac, execute the packaged SLVersionChecker directly rather than invoking
system Python on our manually-bundled scripts.

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

This reverts commit 0a745b47.

Fix for SL-16208 Introduce a canonical_repo tag into the autobuild.xml file for new 3p repos - no new code introduced in this pass

This reverts commit 203ea3a7, reversing
changes made to 8e3f0778.

# Conflicts:
#	indra/newview/llmachineid.cpp

This reverts commit adb72456.