No Explicit Python Version Dependency In Package Definitions?

[Phil]

Hi,

It seems standard not to declare python2 or python3 as a dependency on
python package definitions - however other dependent python libraries are 
stated.

eg python-scipy will declare dependencies on python-numpy and
python-matplotlib - but not on a specific version of python package
required to use it.

I'm guessing this is to avoid tying packages to specific python
releases, but I'm curious about the mechanics.

It looks like 'package-with-python2' might be used to allow us to
distinguish between python2 and python3, but ignoring the python2 case,
I have the following python3 questions:

Can we can install python-scipy without installing python3, given python
isn't an explicit dependency in the package?

Which site-packages directory under what python3 version will be used?
A quick check looks that /path/to/profile/lib/python3.8/site-packages is
currently used but what makes the decision to put them under python3.8 -
especially if python3.8 isn't installed in the profile?

What happens if Guix upgrades python3 from 3.8 -> 3.9?  How are packages
already installed under the 3.8, moved to the new 3.9 python version, is this 
seamless?

If I'm using Guix on top of a foreign distro and don't have python3
installed as part of Guix, will my python-scipy end-up installed for my
foreign distro's python install?

Any pointers welcome!

Thanks,
Phil.