Add support for python3 to the recipes

* Remove dependency on python module.
* Detect python3 based on shebank in build.py.
* Allow python_version property to override the detected python.
* Run all python steps with python3 when python3 is selected.

Change-Id: I9237135ec7796d364edcbfb73032bbf27c25248e
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/192360
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
39 files changed