use dynamic default to delay import of jupyter_client optional dependency

[minrk]

using a string as a static default doesn't defer import (which is not obvious).

closes #556

[minrk]

#499 didn't actually fix #495 because types declared in trait constructors are resolved at instantiation time. This surprises me, and I don't think it's always been the case, but using a dynamic default actually does resolve only when the value is requested.

[mpacer]

Looks good to me.

[damianavila]

#499 didn't actually fix #495 because types declared in trait constructors are resolved at instantiation time. This surprises me, and I don't think it's always been the case, but using a dynamic default actually does resolve only when the value is requested.

Thanks for the explanation. Looks good to me as well and tested successfully.