Python 2.7 support broken with latest pyrsistent release

[evandentid]

pyrsistent release 0.17.0 no longer supports py2

https://github.com/tobgu/pyrsistent/blob/master/CHANGES.txt#L10-L14

Version set here: https://github.com/Julian/jsonschema/blob/master/setup.cfg#L31

[Julian]

Hi, this is a dupe of #732

Have a look at the upstream ticket there in pyrsistent which is where the fix should go.