Bootstrap script incompatible with python3

[bartoldeman]

As observed in #3017 bootstrap_eb.py is not compatible with Python3. It relies on /usr/bin/env python being Python 2 (which is still the case on most but not all Linux distributions).

Fixing that is not a simple case of translating the file, as it included a base64'ed gzipped setuptools helper that is outdated.

One way that seems to work for stage1 is something akin too:

wget https://bootstrap.pypa.io/get-pip.py
export EB_TMPLOC=<sometmplocation like /tmp/tmpRG7iSQ/eb_stage1>
export PYTHONPATH=$EB_TMPLOC
python3 get-pip.py --prefix=$EB_TMPLOC
$EB_TMPLOC/bin/pip --prefix=$EB_TMPLOC easybuild

after that it can do stage2 as usual, since eb is installed now.

I'm just floating this as an idea since it's a complete redesign of the stage1 of the bootstrap.

[boegel]

I started working on this, and it's indeed a bit more work that just porting the script itself.

The good thing is that since EasyBuild 4.0 no longer requires setuptools at all, we can actually move get rid of the base64'd distribute installation script (and just check if setuptools is available in case somebody installs an older version of EasyBuild, rather than trying to install it ourselves which is a bit of a desperate attempt anyway).

That also means no longer using easy_install during stage 1 (which is provided by setuptools), we should just use setup.py install (which implies downloading and unpacking the source tarballs first).

Kickstarted the effort in #3028...