Test all backends together in CI

[RMeli]

Description

Thanks to PR #107 spyrmsd now supports switching between backends and PR #111 added support for an additional backend. With multiple OSes and Python versions, the CI matrix has become large, and it can be reduced by testing all backends in one go.

This PR:

• Adds a session fixture setting the backend, parametrised on all available backends
• Changes some test parametrisation that were not playing nicely with re-running the tests with another backend
• Removes the test/molecules.py file (also not playing nicely with re-running the tests) in favour of fixtures
• Remove the backend-specific conda environments
• Move backend tests to a separate file

Closes #110

Checklist

• Tests
• Changelog

[RMeli]

Cycling CI for #119

[RMeli]

Parametrisations of the following form

@pytest.mark.parametrize(
    "G1, G2",
    [
        *[(graph.lattice(n, n), graph.lattice(n, n)) for n in range(2, 5)],
        *[(graph.cycle(n), graph.cycle(n)) for n in range(2, 5)],
    ],
)

have been changed to be parametrised on n, since they are cached and therefore don't work when changing backend (the graph remain of the first backend).