Pin to jsonschema < 4.0

[terencehonles]

jsonschema 4.0 deprecates RefResolver.in_scope and breaks RefResolver.resolve for Spec.from_dict.

[terencehonles]

I have https://github.com/terencehonles/bravado-core/tree/fix-deprecation-warning-for-RefResolver.in_scope which fixes the deprecation warning, but when testing the change I noticed there was an error during resolving. I checked and this looks like it only affects Spec.from_dict (I skipped all tests that go down that code path and the suite passed). It doesn't appear to be straightforward to find out what changed since I'm not familiar with this package or jsonschema but it looks like it's related to python-jsonschema/jsonschema#782

[terencehonles]

The AppVeyor failure looks like it's possibly a Windows only issue? I cannot reproduce with platform linux2 -- Python 2.7.18, pytest-4.6.11, py-1.10.0, pluggy-0.13.1

[terencehonles]

#385 should probably be considered instead of this one