* Update default containers to 3.4.0.
The 3.4.0 containers use Python 3.6 (the system Python) for `/usr/bin/python3`.
Python 3.9 continues to be the default Python version selected by `ansible-test` for these containers.
* Fix shebang on build-ansible.py.
Using `python` instead of `python3` allows `ansible-test` python interception and requirements install to function.