[FEATURE] Add python version metadata to `package.xml`

[luzpaz]

Is your feature request related to a problem? Please describe. Addon Manager will make it easier for everyone if it checked minimum/maximum python version for an addon. Example: https://forum.freecadweb.org/viewtopic.php?p=591793#p591793

Describe the solution you'd like Per @chennes

I think the way to implement it is to allow you to specify a dependency on Python, and then write the code to support the version info (which the metadata supports, but I haven't actually implemented yet). So you'd have something like:

<depend version_gte="3.7">python</depend>

This won't work right now because the Addon Manager will assume that "python" is some sort of pip-installable package. I'd just have to manually code in support for it, but I've already done that for Plot, so it's not a huge departure.