Config: check for conda constraints and raise a build error

[stsewd]

Now we have docs for this https://github.com/readthedocs/readthedocs.org/issues/5388 but it would be better to tell users before building their docs.

This is a breaking change, since it will fail for docs that were passing. Maybe we could use a feature flag.

[humitos]

This is a breaking change, since it will fail for docs that were passing

I'd say that it's fine to introduce this and make those builds to fail. They are not working as the user expects anyways, and I don't think they would be too many.

[humitos]

We don't support python.version not python.system_packages anymore. The only missing thing here would be to raise an exception when build.tools.python is Conda or Mamba and python.install is used together.