If true, `py_binary` and `py_test` targets that do not set their
`python_version` (or `default_python_version`) attribute will default to
PY3 rather than to PY2. It is an error to set this flag without also
enabling `--incompatible_allow_python_version_transitions`. If you set this
flag it is also recommended to set `--
incompatible_py2_outputs_are_suffixed`.