Update dependency nbconvert to v6

[renovate]

This PR contains the following updates:

Package	Update	Change
nbconvert (source)	major	>=5.3.1,<6.0 -> >=5.3.1,<6.1

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Release Notes

jupyter/nbconvert

v6.0.7

Compare Source

v6.0.6

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v6.0.5

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v6.0.4

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v6.0.3

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v6.0.2

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v6.0.1

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v6.0.0

Compare Source

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[codecov-io]

Codecov Report

Merging #122 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #122   +/-   ##
=======================================
  Coverage   80.46%   80.46%           
=======================================
  Files          29       29           
  Lines        1602     1602           
  Branches      217      217           
=======================================
  Hits         1289     1289           
  Misses        262      262           
  Partials       51       51           

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