support for tox with dotted env

[gpongelli]

as per comment , I'm opening a new issue regarding TOX env with dot inside:

[tox:tox]
isolated_build = true
envlist = lint, format, py3.8, py3.9, py3.10, py3.11, build

looking at the code, a new elif branch should be put here, something like:

    elif env.startswith('py') and '.' in env:
        return Version.from_string(f'{env[2]}.{env[4:]}')

next week I could start a PR on this topic.