Keep compatible with earlier pip versions.
Fixes https://github.com/buildout/buildout/issues/598

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Solution: use `python -m pip`

Solution: implement and document it

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Solution: check value explicitely and open corresponding files

Solution: check for existence of namespace directories

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Solution: install proper pip

Solution: add jobs to GH actions and to CircleCI

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Solution: remove useless configs

Solution: add explicit error message

Solution: hide them in doctests

Solution: fix verbosity detection when calling pip

Solution: pass it properly to buildout via a call to sys.exit

Solution: proper os.path.isfile each time it is needed in
make_egg_after_pip_install

some variable rename

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This happens when installing packages with C extensions that do not have
the proper binary wheel available.

Solution: append platform to the name of egg directories
to work around a bug in pkg_resources that fails to parse
python version if platform is missing

Solution: install debian package python3-setuptools, -wheel and -coverage.

The one used before can't be installed any more by current `setuptools` versions because it uses `2to3` which is no longer supported.

Solution: update to latest versions ;-)

Solution: use set() to avoid adding identic paths multiple times

add some debugging

Solution: stop running 2.7

Solution: run tests as another user

Solution: change imports appropriately

This fixes #483