Upgrade python files to Python 3

[jonespm]

Python 2.7 has been long deprecated and likely won't be supported by future packages and dependencies. These files should be updated to use Python 3.

[mpellicer]

#8 requires this work, you're right!

[jonespm]

I'm not sure if it's required since I didn't actually see the library used directly.

I think this probably might have been easier to upgrade if it was using the python library rather than calling the command line tx CLI command though. And upgrading to Python 3 is something that should happen as 2.7 has not had any fixes for 3 years.

[jonespm]

I can assign myself to work on this but I don't expect to have time for at least the next month or so. And if it's running fine for now it seems like lower priority.

[mpellicer]

Don't worry @jonespm , not required, but nice to have!