Comment 1 for bug 1954295

py3.9 introduced `str.removeprefix`
https://docs.python.org/3.9/library/stdtypes.html#str.removeprefix

 > return charm_url.removeprefix("cs:")
 E AttributeError: 'str' object has no attribute 'removeprefix'

the jenkins runs are py38. Will adjust the test for more py38 compat