Unpin sqlachemy (not needed with nbgrader 0.6.1)

The pinning is incompatible with Python 3.8 which is the default with the recent jupyter/xxx docker images. I believe it's not required anymore since the bug was fixed in https://github.com/jupyter/nbgrader/pull/1080 while nbgrader 0.6.1 was released in november.

(I did not test it though, nor actually checked that the commit was there).