Adds input of NotebookNode object directly

[dcnadler]

What does this PR do?

User can pass an in-memory nbformat.NotebookNode object to pm.execute_notebook instead of requiring a path.

Related to issue #444

This adds a NotebookNodeHandler to read the input object.

A bit of refactoring was also needed to move code that expected a string:

• Stdin/Stdout was pulled out into its own handler
• The check and warning for .ipynb and .json file extensions by PapermillIO read/write was moved into get_handler (this also made the code more DRY)

Happy to take feedback, thanks!

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[codecov]

Codecov Report

Merging #670 (17be0f5) into main (98013f0) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
+ Coverage   91.72%   91.90%   +0.17%     
==========================================
  Files          17       17              
  Lines        1583     1605      +22     
==========================================
+ Hits         1452     1475      +23     
+ Misses        131      130       -1     

Impacted Files	Coverage Δ
papermill/engines.py	98.37% <100.00%> (+0.09%)	⬆️
papermill/execute.py	100.00% <100.00%> (ø)
papermill/iorw.py	81.15% <100.00%> (+0.91%)	⬆️
papermill/parameterize.py	98.00% <100.00%> (+0.04%)	⬆️

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[rohitsanj]

Looks good! Thanks for the clean refactor.