Incompatible with graphql-core==3.0.0

[crunk1]

graphql-core 3.0.0 was just released 3 days ago and now my builds are breaking. I have had to specify graphql-core==2.2.1 in my requirements.txt to fix this. I'm guessing that the Flask-Graphql module dependencies need updated to prevent graqhql-core>=3.

Python: 3.7.5
Flask-Graphql: 2.0.0
graphql-core: 3.0.0

Truncated stack trace:

...
  File "/opt/python3.7/lib/python3.7/site-packages/flask_graphql/__init__.py", line 1, in <module>
    from .blueprint import GraphQL
  File "/opt/python3.7/lib/python3.7/site-packages/flask_graphql/blueprint.py", line 5, in <module>
    from .graphqlview import GraphQLView
  File "/opt/python3.7/lib/python3.7/site-packages/flask_graphql/graphqlview.py", line 7, in <module>
    from graphql_server import (HttpQueryError, default_format_error,
  File "/opt/python3.7/lib/python3.7/site-packages/graphql_server/__init__.py", line 5, in <module>
    from graphql import get_default_backend
ImportError: cannot import name 'get_default_backend' from 'graphql' (/opt/python3.7/lib/python3.7/site-packages/graphql/__init__.py)

[ivancarrancho]

Hi, This MR fix it
#65

[jkimbo]

Should be fixed with v2.0.1: https://github.com/graphql-python/flask-graphql/releases/tag/v2.0.1

Sorry for the delay