Excise Python2 requirements

[Dan Bonachea]

Python2 is end-of-life on Jan 1 2020, and will no longer receive security updates.

NERSC has just announced the cori install of Python2 will disappear in 2020, and will not be installed/supported at all on future NERSC systems.

Apple has also announced that Python2 will disappear in future versions of macOS (issue #255).

While the EOL Python2 software will remain available, continuing to require it for UPC++ means putting users in the uncomfortable position of installing a large, unsupported software package, which will likely suffer bit rot and possibly even active security bugs. UPC++ needs to be updated to stop requiring Python2, and I think this needs to happen by our March 2020 release.

upcxx-run should already be Python3-compliant, so this issue mostly applies to nobs. Fixing this means either updating nobs to work in Python3 or replacing it entirely with a standard build system. We should discuss and make plans to resolve this.

[Paul Hargrove]

My preference would be to see nobs replaced with something that is maintainable by more than just its original author.
That is an observation entirely independent of the EOL status of python2.

[Dan Bonachea]

This was resolved in pull request #134 which removes our dependence on Python 2.7