is it possible to use multiple python versions with pants? E.g., mark some libraries/binaries as using 2.7, others as using 3.6, in the same repo?

I haven’t used this myself, but take a look at the python_binary compatibility option: https://www.pantsbuild.org/build_dictionary.html I believe if you want to build binaries for python 2 & 3 you could have two python_binary targets, each with the same options, but with each target customizing the “compatibility” arg.

Yes, you can. Travis is correct. Pants will plumb the interpreter constraints defined in the target's compatibility field through to the produced pex, and the pex will search PEXRC or $PATH at runtime for an interpreter that satisfies the constraints.