Add tests for all poetry versions between 1.5 and 1.8

[enpaul]

Since we are relying on semi-internal poetry API components we should test that the components we are using exist and are compatible in every version of Poetry we support. I think this can be done fairly easily by simply installing an unpinned version of poetry into a toxenv and then re-running pytest and mypy. An example of such a toxenv is below:

[testenv:poetry15]
require_locked_deps = false
poetry_dep_groups =
    dev
deps =
    poetry>=1.5.0,<1.6.0
commands =
    pytest {toxinidir}/tests/
    mypy {toxinidir}/tox_poetry_installer/

This should raise warnings in the event that poetry~1.5 has an incompatibility, even if the locked version of poetry is 1.8+