I'm getting pretty stumped with a solve for the issue. Effectively I'm trying to implement something that can more accurately represent the "environment markers" present in python package requirements.
I came up with an example that I think would be impossible to replicate in Rez currently:

[
     'importlib-metadata ; python_version < "3.8"',
     'click ; python_version > "3.6"'
]

We can't represent these 2 package requirements as variants like this:

variants = [
    ["importlib-metadata", "python<3.8"],
    ["click", "python>3.6"]
]

because Rez will only select one of these variants at resolve time, when we may need both (ex. we request Python 3.7)
It almost seems like we need to replicate the package markers functionality in Rez packages. Unless there's some other Rez functionality I don't know about yet.
That being said, maybe we could do something where if there 2 or more non mutually exclusive requirements like there is above, we just do the old rez-pip1 solve where we lock each variant down to a specific <major>.<minor> python version? Ex.

variants = [
    ["importlib-metadata", "click", "python-3.7+<3.8"],
    ["click", "python-3.8+<3.9"]
]

Any new <major>.<minor> versions would not be supported here though and you'd have to do a new rez-pip for each new Python version as you need support for it.

I see, I don't know how to cleanly and easily solve this. The only way I can think of right now would be by "exploding" the matrix:

variants = [
    ["importlib-metadata", "python<3.8"],
    ["click", "python>3.6"],
    ["click", "importlib-metadata", "python>=3.6|<3.8"]
]

Alternatively, we could create a late bound variants functions... Both options would be hard to implement and both options wouldn't be reliable I think.

The last option is to implement markers in rez to have a more declarative option than @late. AcademySoftwareFoundation/rez#1503 would be of interest I think. One of my arguments in that discussion was that by making the new variants generic, we could introduce new features. For example, we were thinking about:

variants = [
     Variant("maya", requires=["foo", "spam", "maya"], private_build_requires=["maya_devkit"]),
     Variant("houdini", requires=["foo", "houdini"],
     ...
]

If we take this and go even further, we could have

requires = [
    Dep("python"),
    Dep("click", when="python>=3.6"),
    Dep("importlib-metadata", when="python<3.8"),
]

This would be a major change and it would require major changes to the rez codebase though...