Flowpipe

Add missing loading of module when deserializing nodes from files

- Dropped Py2 support
- Cleanup of Code
- Fix for Pickling Error 168
- FunctionNodes can now be used directly without going through the Node decorator

Add previously deleted Plug._sub_plugs method back in for backwards compatibility.

**Full Changelog**: https://github.com/PaulSchweizer/flowpipe/compare/v0.10.0...v0.10.1

- Addopting poetry
- Code quality

Convenient feature: Make input plugs of nodes available directly on the graph for easy access.

This minor release brings two changes:
* A bug was fixed in event handling that lead to callbacks being triggered multiple times.
* Better debug logging for the threaded evaluation of the graph was added.