Prevent running gh-actions twice for push/pull.

Copied from the zopefoundation repositories.
See https://github.com/zopefoundation/meta/issues/145
and the final version at https://github.com/zopefoundation/meta/blob/9c24f7380973de989c5132ec9f28fa71f1397f20/config/c-code/tests.yml.j2#L93

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Solution: remove it now that it seems to be correct

Solution: ignore until fixable

Solution: be explicit instead of relying on auto-discovery

Solution: workaround pip _distutils_hack

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Solution: fix it

btw, exclude CI only file from sdist

Solution: use explicit strings

Solution: update

Solution: stop testing it

Solution: be explicit where to find it

Solution: rewrite in easy_install

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