As part of the whl_library rule, we run a python script (packaged here as whltool.par) to generate py_library instances for each wheel. One of the functions of running that script is to parse the wheels' declared dependencies. In some cases, the sets of dependencies may vary depending on what python version is being used, even for a wheel file that is compatible with both python 2 and 3. This pull request adds an argument to the whl_library rule allowing the user to specify the desired version of python to run the whltool script - in my case, I pass the value python3. It defaults to the original behavior of just running python by itself.

To give a concrete example of where this can come up, the cryptography package relies on a backported package when used under python<3.4, but does not require that backport for newer python versions: https://github.com/pyca/cryptography/blob/master/setup.py#L239-L240 . When attempting to install this library with python 3.7, it's not possible to find that backported library, and so we can't resolve the dependencies.

I'm very open to renaming things here if desired, or to (say) attempt to resolve the path to the python interpreter using something like https://docs.bazel.build/versions/master/skylark/lib/repository_ctx.html#which. I elected to use python_target so that the user would understand that this should correspond to the target python version that will eventually run whatever code relies on the whl_library.