Move tests out of package and into top-level directory

[jdufresne]

This follows the "Tests outside application code" pattern documented in
the pytest guides. See:
https://docs.pytest.org/en/7.1.x/explanation/goodpractices.html#tests-outside-application-code

This has the following benefits:

• Your tests can run against an installed version after executing pip
  install ..

• Your tests can run against the local copy with an editable install
  after executing pip install --editable ..

• If you don’t use an editable install and are relying on the fact
  that Python by default puts the current directory in sys.path to
  import your package, you can execute python -m pytest to execute the
  tests against the local copy directly, without using pip.

This keeps tests separate from actual library code. End users don't need
the tests mixed in with the installed packages. Instead, tests are only
interesting to library developers and packagers. The tests remain
available, just moved.

[larsoner]

I'm not totally convinced. Most scientific Python packages (numpy, scipy, scikit-learn, etc.) package within the module. Often tests change along with the library (exceptions, new tests, etc.) so I'm not sure there is too much value added here for "running tests against other versions", which seems to be the primary motivation in the pytest docs.

[DimitriPapadopoulos]

Moving to pyproject.toml and changes in tests require resolving conflicts, probably complex ones.

Also, it looks like this change did not convince: it's one set of conventions against another. Why change and not stick with the initial conventions?