STYLE: src/Python: Improve readability using explicit import

It improves the readability of the examples and help the reader
understand where objects come from.

See also See https://stackoverflow.com/a/2386740/1539918


Otherwise, from "The Zen of Python":

  Explicit is better than implicit.

See https://www.python.org/dev/peps/pep-0020/


Former-commit-id: 7c14bcd8