text: Title: AttributeError: module 'pykwalify.core' has no attribute 'yaml'
  Body: Hello, it looks like pykwalify has had a new release (1.8) which is breaking my tavern build with the following error:
  ```
  ve/lib/python3.8/site-packages/tavern/schemas/files.py:16: in <module>
      core.yaml.safe_load = functools.partial(yaml.load, Loader=IncludeLoader)
  E   AttributeError: module 'pykwalify.core' has no attribute 'yaml'
  ```
  
  It looks like pykwalify has dropped/changed their default yaml parser which may be the culprit?
  
  Could we maybe get tavern pinned to pykwalify 1.7 until it's fixed?
  
  https://github.com/Grokzen/pykwalify/releases
  https://github.com/taverntesting/tavern/blob/master/setup.cfg#L34