By default, DeprecationWarning and PendingDeprecationWarning, and
ImportWarning are ignored, we want to see those while running tests, so
we reset the warnings module to use "default" behavior for all warnings.

https://docs.python.org/3/library/warnings.html#default-warning-filter
https://docs.python.org/2.7/library/warnings.html#updating-code-for-new-versions-of-python

This also implements the recommendation from
https://docs.python.org/3/library/warnings.html#overriding-the-default-filter

> Developers of test runners for Python code are advised to instead
> ensure that all warnings are displayed by default for the code under
> test, using code like:

    import sys

    if not sys.warnoptions:
        import os, warnings
        warnings.simplefilter("default") # Change the filter in this process
        os.environ["PYTHONWARNINGS"] = "default" # Also affect subprocesses

This also extend the "Run Live Test" action to allow controlling the
warnings filter, but on python2 this does not work so well, once a
warning was ignored, later switching to "always" or "error" does not
process the warning again ( it might be because of
 https://bugs.python.org/issue4180 )