[Design Doc] Controlling the Python 2/3 mode

[Jon Brandvein]

Python 2/3 mode selection has a number of issues. While trying to solve some of them, it turned out that the way the Python mode is represented became a bit of a blocker.

This design doc explains the current model and outlines a proposed simplification that should help significantly. Tracking bug here.

Note that this affects things like srcs_version, default_python_version, and --force_python. Actually controlling what interpreter gets invoked (py_runtime, --python_top) is a separate issue that needs fixing.

[ai...@google.com]

ptr to doc:  https://github.com/bazelbuild/rules_python/blob/master/proposals/2018-10-25-selecting-between-python-2-and-3.md