write_standard_name_table.py doesn't work with python 3.7+

[nusbaume]

Currently the write_standard_name_table.py script will not work with later (3.7+) versions of python due to an incompatibility in the subprocess flags used in the call_command function contained within xml_tools.py. This can be resolved by simply updating the xml_tools.py file to the latest version used in the CCPP-framework.

[nusbaume]

I should note that entirely updating the xml_tools.py file would require additional CCPP framework source files to be added to this repo. Thus it might make sense to either update just the call_command function to avoid this conflict, or to bring in the CCPP-framework itself as a submodule.