stop using bootstrap script to install EasyBuild, just use 'pip install'

[boegel]

As mentioned in #1593, the .spec file for installing EasyBuild should be updated to stop using the bootstrap script, which is deprecated since June 2021.

For quite a while now (since EasyBuild v4.0.0), EasyBuild is a pretty standard Python package that can be installed with standard Python tools like pip install.

The only peculiar thing about the easybuild Python package is that it's a meta-package: it basically only pulls in the 3 packages that actually provide the EasyBuild code and accompanying easyconfigs (build recipes, in some sense): easybuild-framework, easybuild-easyblocks, easybuild-easyconfigs.

[adrianreber]

Thanks for opening this tracking bug. I looked at it for a short time and the three packages was what confused me. There are a lot of macros to handle Python packages but with easybuild being a meta-package it was not immediately clear what to do.

Let's try to get it ready for the 2.7 release. We are in the middle of the 2.6 work and I do not think we want to introduce bigger changes at this point.

[adrianreber]

Fixed in #1621