DeprecationWarning: inspect.getargspec() is deprecated since Python 3.0, use inspect.signature() or inspect.getfullargspec()

[README] add testing instructions

Add pytest-subtests to dev requirements This pytest plugin adds support for unittest's TestCase.subTest (for Python >= 3.4). Since we we now only support python3 and since we use a fair amount of these, let's benefit of better reporting by using this plugin.