python3: Mark all modules as PY3 compatible

Bug: 465
Change-Id: I303985673c286188d8cad889790d1904ace8b957
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/59285
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Oliver Newman <olivernewman@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
24 files changed