Make absl py3_mode config setting work with Bazel's new python version behavior.
Make absl py3_mode config setting work with Bazel's new python version behavior.

How Bazel selects and determines the Python 3 mode is changing. To remain
compatible, switch from using the --force_python flag to the newer config
setting method.

This also makes Bazel 0.23 with --all_incompatible_flags work.

PiperOrigin-RevId: 237345747

abseil/abseil-py,abseil/abseil-py