Check if Python interpreter is finalizing prior to logging

In our logger, we can potentially try to log out a message from the C++ client when the Python interpreter is finalizing. This can cause an "unwanted termination" (fancy speak for crash). Lets check if finalizing (via Py_IsFinalizing) prior to calling pycbc_logger_sink::log_it().

Also, pycbc_logger_sink::convert_log_msg() utilizes Py_None w/o incrementing the PyObject. Need to call Py_INCREF(Py_None) when using. Lets fix that too.