Should fix some common crashes for parcel media

SL-18151 [SEC] MOAP can force multiple floaters open on users screen: The Viewer part of the fix for this JIRA - pulls in the updated Dullahan that exposes the user_gesture/is_redirect flags and uses them to determine what type of 'nav_type' is exchanged with viewer/plugin

Nicky Dasmijn's contribution for zlib-ng Linux builds

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

SL-13297 - Update autobuild.xml for new 3p-slvoice dependency following commit message changes there.

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