enhance 'eb' command to ensure that easybuild.main can be imported before settling on python* command to use

[boegel]

This is a small enhancement for the eb command: only accepted the python* command if it's able to import easybuild.main.

This fixes the following strange/confusing situation, which occurs currently because eb considers python before python3 (unless $EB_PYTHON is set):

# default Python command is Python 2
$ python -V
Python 2.7.18

# use pip3 to install EasyBuild, with the intention to run it via 'python3'
$ pip3 install easybuild

# try running 'eb' fails because default 'python' command is not Python 3
$ eb --version
/usr/bin/python: No module named easybuild

[akesandgren]

LGTM

[akesandgren]

Going in, thanks @boegel!