unpin jsonschema & flasgger

[aschonfeld]

Currently flasgger is pinned to version 0.9.3 which in turn forces jsonschema to a version <3.0.0.

Possible options moving forward:

• find a way to unpin flasgger & jsonschema
• drop flasgger support
• make flasgger available as an extra requirement

FYI, other packages which currently have collisions with the version of jsonschema are:

• sphinx (strictly for documentation purposes)

[aschonfeld]

https://github.com/man-group/dtale/commit/21c15755958f29a4c987d63d7764d583545f4446 moved flasgger to extras_require