Migrate all scripts, symlinks, and installed tools to Python3

[earlephilhower]

Python 2 has less than 5 months left to live (or at least, until all support ends)
https://pythonclock.org/

#6364 had an issue with a symlink to Python 2 being installed in the tools directory, which is one part but not all.

We need to:
. Update all script refs from /usr/bin/python to /usr/bin/python3
. Update the tools python-wrapper.tgz to point to python3
. Update the Windows standalone installation to python3
. Update platform.txt/etc. to ensure it's using the right python verison
. Validate things still work.

It's not as simple as changing scripts to #/usr/bin/env python, either, as that only works if there is a Python2 installation somewhere. Newer OS releases may not install Python2 by default.

[TD-er]

And what about using virtualenv as wrapper?
Then you can also have a very well contained set of included libraries.
Not sure how easy virtualenv is setup in all OS'es (e.g. Windows), but on Linux I use it all the time.

This also allows to create a stand-alone Python 2.7 environment if conversion takes more time than expected.

[earlephilhower]

I think the point is that Python 2.7 is dead as of January 1, 2020, so virtualenv's not really a solution. Without the python 2 executable, the virtualenv would run under the Python 3 interpreter, in fact. I don't think there's any issue with scripts we're using, they should already run fine under Py3.

The standalone Python we're using for Windows is kind of a virtualenv. We install the needed libraries right in the core directories, and it only uses PYCs from inside the installation dir, not system-wide ones (assuming there's some system-wide Python installed on the user's Windows).