Call out the lack of support for python 2

[jarshwah]

I installed the plugin into our composer instance, and failures caused more failures (syntax error with * args) because our composer is still running python 2 (1.9).

I realise python 2 is out of support, and I'm not asking for it, but it might be handy to call out the plugin only works on python 3 :)

[mikeclarke]

Ah - it should work on python 2, I opened #35 as a potential fix.

[jarshwah]

Sorry I didn't provide more context - I couldn't actually find the error, coworkers told me of it and restarted the tasks after I removed the plugin.

Is there any good way to test the plugin working without waiting on a task to crash? :)