Add tox testing

[ElliottKasoar]

Resolves #26

We may want to wait to see how @federicazanca has implemented tox for aiida-mlip, but this seems to work as expected e.g. using tox run -e ALL.

To address the questions in stfc/aiida-mlip#59, I think the default tox run will only run the pytest section with Python 3.10, but, using -e, all tests, including the range of Python versions listed, can be tested individually/together.

Note: with the current set up (poetry install --no-root --sync)

tox will create an sdist package of the project and uses pip to install it in a fresh environment. Thus, dependencies are resolved by pip in the first place. But afterwards we run Poetry, which will install the locked dependencies into the environment.

There are a couple of alternatives, including allowing tox to install everything, or running poetry install --with dev etc. with skip_install = true, in which case:

tox will not do any install. Poetry installs all the dependencies and the current package in editable mode. Thus, tests are running against the local files and not the built and installed package.

(See https://python-poetry.org/docs/faq#is-tox-supported)

This might be worth looking into this more in aiida-mlip @federicazanca, as we have more dependencies already set up to test.

Also note we already have

[build-system]
requires = ["poetry-core>=1.0.0"]
build-backend = "poetry.core.masonry.api"

in pyproject.toml, so tox is able to build the package using poetry as expected, and isolated_build is now always used, so this does not need to be added.