Some tweaks to get closer to Python 3 compat

Convert to use idioms that work for both python 3 and python 2.6+ and
ensure that a suitable version of dependencies is included for python 3
compatibility.

Update python-jenkins to 0.3.3 as the earliest version that supports
python 3 without any known regressions. Add an extra parser check for
missing 'command' due to changes in how argparse works under python 3.

Where contents should be retained, to access the first element of a dict
in both python 2 and 3, 'next(iter(dict.items()))' is used as the
standard idiom to replace 'dict.items()[0]' as 'items()' returns an
iterator in python 3 which cannot be indexed. Using 'next(iter(..))'
allows for both lists and iterators to be passed in without unnecessary
conversion of iterators to lists which would be true of
'list(dict.items())[0]'.

Alternatively, where further access to the data is not required,
'dict.popitem()' is used.

Change-Id: If4b35e2ceee8239379700e22eb79a3eaa04d6f0f
Merge "Some tweaks to get closer to Python 3 compat"