This is necessary because at various points we cross over between Python and shell scripts (because the migration in bug 1247781 isn't complete yet). The shell scripts generally use the PYTHON3 environment variable to determine which Python to use. In the absence of this environment variable, it will generally use the Python at the head of the PATH, which is usually the incorrect behavior.

mach configure currently runs the equivalent to make -f client.mk.
This is history, and essentially does the following:

• Create configure and js/src/configure from configure.in and
  js/src/configure.in respectively.
• Create the objdir.
• Run configure from the objdir.

The configure script is, nowadays, only really used as a means to set
OLD_CONFIGURE (and also for people who want to run configure,
literally, as in the configure; make workflow). mach configure
actually doesn't need it. Neither does recursing into js/src require
js/src/configure, since bug 1520340 (and now as of bug 1669633, we
don't even recurse).

Because configure.py can actually derive OLD_CONFIGURE on its own
(except for js/src/configure, but mach configure doesn't run that),
we don't really need configure for mach configure.

So all in all, we're at a point in history where it's straightforward to
just initiate configure.py from mach configure, so we just do that.

And in the hypothetical case where the mach configure code is somehow
running in python2, we get the mach virtualenv python3 and use it to
execute configure.py.

Backed out for bustages on configure.py

Backout link: https://hg.mozilla.org/integration/autoland/rev/eb901bc7213ed80951e28c5bd5330655d6e29c85
Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=319055128&repo=autoland&lineNumber=1334

Please also check:

1. https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=319057420&repo=autoland&lineNumber=2141
2. https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=319072637&repo=autoland&lineNumber=5621

https://hg.mozilla.org/mozilla-central/rev/8af2e5440a95