Do not raise a fatal exception for an old Python interpreter

[d-w-moore]

The following line will currently abort a Python interpreter of a version older than irods.minimum_compatible_python:

python-irodsclient/irods/__init__.py

Line 10 in 545997d

raise RuntimeError(version_message)

The exception type is too broad to simply catch in reponse to import irods, so we could either:

1. change the exception to something like PythonVersionTooEarly, or
2. log (at the WARNING level) the Python Interpreter loudly as too early to claim compatibility with the PRC (or use the word deprecated, if it is more apt and convenient)

I like (2) better. It's a slightly deeper change, but I think (1) is error prone. An import is done only once, and you'd have to be sure to catch the right instance of the import, which is hard for all cases. And I don't think anyone is depending on the Exception behavior as an operational tool to prematurely abort their applications.