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

Make validate_schemas show best-guess for schema script, to help build engineers

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

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.

SL-38184 Make python scripts in linden/scripts run in place without PYTHONPATH or symlinks.  Reviewed with Donovan.