remove nbconvert < 6 compat patches

[chrispyles]

Closes #777

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 82.29%. Comparing base (afa146d) to head (8dbf900).

Files	Patch %	Lines
otter/export/exporters/via_html.py	20.00%	4 Missing ⚠️
otter/export/exporters/__init__.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             beta     #785      +/-   ##
==========================================
+ Coverage   82.21%   82.29%   +0.07%     
==========================================
  Files          64       64              
  Lines        4398     4389       -9     
==========================================
- Hits         3616     3612       -4     
+ Misses        782      777       -5     

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