python-tox.yml: allow overriding tox requirement specifier

In a teams thread it was noted that the most recent version of tox cannot be installed alongside the most recent version of poetry.

Although it is possible to pin the version of poetry by specifying a version in TOX_ADDITIONAL_REQUIREMENTS, one cannot pin the version of tox itself.

Add a new variable, TOX_REQUIREMENT, which defaults to tox but which can be set by downstream jobs to pin a previous version of tox if a similar situation happens again.

Found by @mk2155