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

following promotion of DRTVWR-520

This reverts commit adb72456.

# Conflicts:
#	autobuild.xml
#	build.sh
#	indra/CMakeLists.txt
#	indra/newview/CMakeLists.txt
#	indra/newview/llappviewermacosx.cpp
#	indra/newview/llappviewerwin32.h
#	indra/newview/viewer_manifest.py
#	indra/win_crash_logger/llcrashloggerwindows.cpp

following promotion of DRTVWR-531

following promotion of DRTVWR-516

This reverts commit efc8b268.

# Conflicts:
#	indra/newview/app_settings/settings.xml
#	indra/newview/llvoicevivox.cpp

following promotion of DRTVWR-518

win_crash_logger will be fully removed in D520, for now just fixing local relwithdebinfo build

Fix for: SL-15422 Update version of CEF used in the Viewer to 91.0.4472.101. Also includes a tweak to viewer_manifest.py to handle some CEF file changes"