Require Python 3.6+ for running Python-based tests

Require Python 3.6+ for running Python-based tests

configure.ac currently requires Python 3.4 for running Python-based
system tests.  Meanwhile, there are some features in Python 3.6+ that we
would like to use for making our Python code cleaner (e.g. f-strings).
Update the minimum Python version required for running Python-based
system tests to 3.6, noting that:

  - Python 3.4 has reached end-of-life on March 18th, 2019.
  - Python 3.5 has reached end-of-life on September 5th, 2020.