subprocess_middleware workers.py uses log.warn rather than log.warning

[netsettler]

In testing some cgap-portal code under github actions, I saw this DeprecationWarning:

  /home/runner/.cache/pypoetry/virtualenvs/encoded-e-hjmLy7-py3.6/lib/python3.6/site-packages/subprocess_middleware/worker.py:124: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
    log.warn('Retrying: %r\n%s', e, errout)

The file https://github.com/lrowe/subprocess_middleware/blob/master/src/subprocess_middleware/worker.py#L124 does indeed appear to have a call to log.warn that should presumably be log.warning, according to logging doc, which says (in the entry for logging.warning:

Note: There is an obsolete function warn which is functionally identical to warning. As warn is deprecated, please do not use it - use warning instead.

I didn't test such a fix, but I did check that indeed logging.warning exists in Python 2.7. I'm using Python 3.6 and it's there as well. I can't speak to Python 3.4, but I imagine it's there. It looks like you're better set up to test this properly, but it sounds like a safe substitution to me.