Update tox versions according to Makefile

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Added a hook to enable a soon-to-be-released buildout extension

provide wheel support.

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Fixes #326

Mainly to disable it when just trying to get distributions for
buildout and setuptools.

That I can no longer reproduce.

NFC

Fixed a bug introduced in 2.6.0:
zc.buildout and its dependeoncies were reported as picked even when
their versions were fixed in a ``versions`` section.  Worse, when the
``update-versions-file`` option was used, the ``versions`` section was
updated needlessly on every run.

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Added a buildout option, ``abi-tag-eggs``

As suggested in: https://github.com/buildout/buildout/pull/325

Also simplified _setup_directories to not bother with the eggs
directory, since that's handled elsewhere.

that, when true, causes the `ABI tag
<https://www.python.org/dev/peps/pep-0425/#abi-tag>`_ for the buildout
environment to be added to the eggs directory name.

This is useful when switching Python implementations (e.g. CPython
vs PyPI or debug builds vs regular builds), especially when
environment differences aren't reflected in egg names.  It also has
the side benefit of making eggs directories smaller, because eggs
for different Python versions are in different directories.

Python 3.6

Update to work with setuptools 34

whatever

Editing while reading

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Do not remove an existing egg.

After a dist is fetched and put into its final place, compile its
python files.  No longer wait with compiling until all dists are in
place.

This avoids calling redo_pyc at the same time by two parallel buildouts.
It might not be so bad, but it seems better (and faster) to avoid it.

See discussion at https://github.com/buildout/buildout/issues/307

When installing an egg to a location that already exists, keep the
current location (directory or file).

This can only happen when the location at first did not exist and this
changed during the buildout run.  We used to remove the previous
location, but this could cause problems when running two buildouts at
the same time, when they try to install the same new egg.

Fixes #307.