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

Result of svn merge -r107256:107258 svn+ssh://svn/svn/user/phoenix/license_2009_merge into trunk. QAR-1165

QAR-1018 Lightweight Windows Setup App (windows-setup-3). svn merge -r102882:102883  svn+ssh://svn.lindenlab.com/svn/linden/branches/windows-setup/windows-setup-3-merge

merge in the fix for template_verifier which avoids multiple
concurrent builds from stomping on each other.

Result of svn merge -r71162:71205 svn+ssh://svn/svn/linden/branches/new-license into release. only changes files which are not deployed or the comments section of code.

export source now detects the prefix rather than assigning it based on filename. fixed up some comments to match our coding standard. added ability to set default license on export which will be useful for the next source drop. SL-56607

Added much more error-recovery logic to template-verifier -- e.g. it retries a few times to fetch the master from the web.  Also I changed the master to live in the system global tempdir, mostly because the build machines had a network problem and I wanted to reduce the chance of that happening.  Not reviewed, but I'm checking it in over the weekend so I can silently fix it if there's yet another weird configuration difference that breaks it on the build machines.

SL-49406: template_verifier.py requires network access to build.  Implemented a 4-hour cache so it hits the network less often overall (saving approx .5 seconds every build), and added fault tolerance for when it fails to fetch the master over the network.  It'll whine at you if it can't fetch it, but even if it can't get the master from the network and doesn't have a cached copy at all, it will still at least do a syntax check on the local template.

Replaced usage of urlopen.read(), which isn't guaranteed to return all of the downloaded bytes, with urlopen.readlines(), which (I believe) does have that guarantee.  This should fix the rash of mysteriously-failing prebuilds.  In the process, added some better error messaging about whether the local template has the parse error, or the master on the network.  Also some reorganization.  Reviewed by Tess and Leyla.

Result of svn merge -r65183:65337 svn+ssh://svn/svn/linden/branches/python-shuffle into release. Also includes untabification of many python files.

svn merge --ignore-ancestry svn+ssh://svn/svn/linden/release@65088 svn+ssh://svn/svn/linden/branches/release-candidate@65078 -> release Paired by Tess and rdw.