RX dependancy conflict for Python 2.7

[Nabellaleen]

It seems RX > 3.0 is not anymore compatible with python 2.7, so I get this error on graphene-sqlalchemy build, as mentionned in graphql-python/graphene-sqlalchemy#177 (comment)

I prepare a PR to fix it

[jeremytubbs]

+1

[Cito]

Fixed in version 2.2.1.