Co-authored-by: Jens Vagelpohl <jens@plyp.com>

Python 3.11 warns:

  DeprecationWarning: It is deprecated to return a value that is not
  None from a test case (<bound method Test_c3_ro.test_complex_diamond
  of <zope.interface.tests.test_ro.Test_c3_ro
  testMethod=test_complex_diamond>>)

Rearrange slightly to avoid this.

Add a regression test for CFLAGS not having -Ofast, which is known to
break things.  See https://github.com/zopefoundation/meta/pull/155 for
reference.

This reverts commit dc2e4b9841185889bdf8b77ee5c28f5b7295578a, which
itself was reverting commit 22a761cc9e6fcfa840582d55a15abda31a9dd2e1,
which was accidentally pushed to git master and is otherwise identical
to this commit.

This reverts commit 22a761cc9e6fcfa840582d55a15abda31a9dd2e1.

This was not supposed to have been pushed directly to master :/

Add a regression test for CFLAGS not having -Ofast, which is known to
break things.  See https://github.com/zopefoundation/meta/pull/155 for
reference.

Co-authored-by: Michael Howitz <mh@gocept.com>

* Add support for Python 3.9.
* Improve coverage.

Make C's __providedBy__ stop ignoring all errors and catch only AttributeError

Fixes #239

There was a similar bug in the Python side where it would ignore a __provides__ of None, unlike the C implementation.
I documented this in the code but not the CHANGES.rst because I can't imagine anyone relying on that.

By moving things from the RHS to the front of the list if they already extend something from the LHS.

Fixes #193