Edit jsonschema version

[chenxing1999]

>>> import swagger_spec_validator
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/xing/Workspace/opensource/swagger_spec_validator/swagger_spec_validator/__init__.py", line 8, in <module>
    from swagger_spec_validator.util import validate_spec_url
  File "/home/xing/Workspace/opensource/swagger_spec_validator/swagger_spec_validator/util.py", line 9, in <module>
    from swagger_spec_validator import validator12
  File "/home/xing/Workspace/opensource/swagger_spec_validator/swagger_spec_validator/validator12.py", line 29, in <module>
    from swagger_spec_validator.ref_validators import default_handlers
  File "/home/xing/Workspace/opensource/swagger_spec_validator/swagger_spec_validator/ref_validators.py", line 14, in <module>
    from jsonschema.compat import iteritems
ModuleNotFoundError: No module named 'jsonschema.compat'

Json schema just release version 4.0 today and cause backward incompatible.

[peterdudfield]

This looks a better way to do it - #146, then it works for python2

[chenxing1999]

Yes, agree