Is your feature request related to a problem? Please describe.
I would like to export venvs for multiple python versions in each of my resolves.

I keep an one instance of every major python version (2.7 and 3.4+) available on my dev box. When resolving the interpreter, pants/pex defaults to the lowest available that meets the given constraints. That's fine, until I want to work with a particular python version.

Describe the solution you'd like
Enhance python venv exports so that they multiple python versions can be exported for each resolve.

The default behavior is fine: one venv for each resolve using the lowest python version in the interpreter constraints.
In the python backend, add two options to the export subsystem that allows a user to:

• limit the exported venvs to a selected (set of) resolve(s), and
  • filter targets to only include the indicated resolve(s)
• define which python interpreter to use.
  • If only one resolve is given: Error if that interpreter does not match the resolve's interpreter constraints.
  • If multiple resolves are given (like the default which does all possible resolves): Warn and ignore any resolves that don't match the given interpreter

Describe alternatives you've considered
It could create a venv for each python version that is (a) available locally, and (b) matches the interpreter constraints.
That seems a bit too shotgun. I'm happy to guide pants as to which venvs I want created.

Additional context

./pants export ::

Here is the pants.toml in the project I'm using to play with this: https://github.com/st2sandbox/st2/blob/pants/pants.toml#L112-L138
It's got multiple resolves, one of which has a different set of intepreter constraints (pants-plugins follows pants' constraints, not my project's constraints).

I don't need tool resolves to have this behavior (at this point).

So, I want to get this matrix of virtualenvs (only some can be exported today, indicated by + in the today column):

This is where the matrix is generated today:

I'm not sure how to add extra cli args for the export goal or how to determine the python version to use here.