Register the markers 'slow' and 'valgrind' in the `conftest.py` file
instead of `pytest.ini` as the latter file is not always present.

* ported the refguide_check module from SciPy for usage
in NumPy docstring execution/ verification; added the
refguide_check run to Azure Mac OS CI

* adjusted NumPy docstrings such that refguide_check passes

Pytest < 3.8 ignored warnings issued during test collection, but that
changed in pytest 3.8 and the method NumPy used to suppress the
PendingDeprecationWarning for matrices no longer worked, or rather, was
exposed as not working. The fix here is to suppress the warning in
pytest.ini and pytesttester.py , which should work as long as the tests
are the only places left where NumPy uses matrices.

An alternate fix is to delay the construction of matrices in the tests
until they are actually run, which has the virtue of test localization
but is a bit more complicated.

See https://github.com/pytest-dev/pytest/issues/3945 for discussion.

* Make PytestTester callable.
* Rename 'timer' to 'durations', corresponding with pytest.
* Offset 'verbose', no entry is now '-q'.
* Move some `ignore` warnings into PytestTester so that they
  affect releases, not just develop.

Include relevant suppression of various warnings.