Solution: stop monitoring setuptools head with 2.7

Solution: update to latest

[ci skip]

[ci skip]

Solution: update MANIFEST.in

Solution: Add missing entry and cleanup wording

Solution: Make it DRY by using ci_build.sh

Solution: Add a job

Solution: Add build on bare machine

Solution: add infrastructure to run both CentOS and Debian

Solution: sort accordingly

When a recipe is developed in a git working copy, .git administrative
database folder should be ignored, like we did with CVS and svn.

fixes #522

Solution: Warn when a script name passed in 'scripts' argument of easy_install.scripts
is not defined in egg entry points.

Solution: show it only once

Solution: use the proper `python-version`

Solution: cache it

Solution: update to latest 3.9.0b1

Solution: sys.exit when pip install fails

Solution: enable them

Solution: update

Solution: fix it

[ci skip]

[ci skip]

Solution: specify 2.7 or >=3.5

Solution: prune zc.recipe.egg_

Solution: cleanup

Solution: sort packages to push site-packages at the end

Solution:
- better detection of need for upgrade
- more protection break by restarting only once

Solution: enable it on Travis, allowing failures

Solution: Remove it, mention `pip`

Solution: run coverage in containers on GH and Travis

Solution: run with Debian container on GH

Solution: pass environment properly, pass USERPROFILE

Solution: fix them

Solution: fix the testing suite speed hack that depends on
environment variables by avoiding - in env var name.

Solution: Add a GH workflow

Solution: monkeypatch `setuptools.packageIndex`
with code from `pip._internal`.

Because it depends on `pip._internal`, it is fragile.
As a first defense against breakage, imports are protected.
In case they would fail, buildout still works.

Solution: Use `pip install`

The meat of the change is in src/zc/buildout/easy_install.py

Some tests had to be refactored a bit to make them more robust.

`tox` is removed to avoid issues due to sharing the same directory
between different virtual environments; `Makefile` enriched to make
per python virtual envs depend on source files.

No need to be afraid of default Python install as we depend on `pip`.
`bootstrap.py` is gone as not needed anymore.