Use the tox native platform testenv setting

[jayvdb]

tox v2.0 introduced platform regex setting, which matches sys.platform and roughly achieves the same as --prepend-ostype-factor.

https://tox.readthedocs.io/en/latest/example/platform.html has an example, but it relies on global envlist to allow invocation of tox without any args. c.f. tox-dev/tox#1144

Using platform should at least be documented in this plugins README, as that may suffice for many users.

Thinking out loud, a --prepend-platform-factor could use the platform testenv settings to automatically add the necessary factors. Tthe plugin currently operates only at global level, not testenv level, but it can easily get these settings from all sections while at global level in order to add them globally.