nbconvert 5.6.0 release

[Matthew Seal]

Hello Jupyter devs and users,

After much effort we've released nbconvert 5.6.0! The biggest feature changes is support for parallelization of nbconvert and improvements to cell executions, though there were many other merged changes. Thanks everyone who contributed and helped get changes made.

For more details see: https://discourse.jupyter.org/t/nbconvert-5-6-0-release/1867

Best,

Jupyter Team