* mac: Use cibuildwheel
* Do not build wheel for PyPy.

* linux: Use manylinux2014
* mac: Drop Python 3.6 too

Also "old" buffer API was removed in Python 3.10

There is no such thing as [variants] in the tox syntax. This resulted
in MSGPACK_PUREPYTHON being unset in the "pure" test environments

This change causes Tox to run the project's setup.py in a virtualenv
(default path is .tox/.package). The required version of Cython is
installed, rather than whatever version is installed system wide.

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