Don't require nbconvert

[takluyver]

Nbconvert is only needed for the execute=True option, which was deprecated in version 0.6. Loading nbconvert may be slow, so we don't want to do it when it's not needed.

A future version of nbparameterise will likely remove the execute= option entirely. Anyone using it should make a separate call to a package like nbclient or princess instead.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e0466fc) 93.33% compared to head (3adb3fc) 92.82%.

Files	Patch %	Lines
nbparameterise/code.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   93.33%   92.82%   -0.52%     
==========================================
  Files           3        3              
  Lines         195      195              
==========================================
- Hits          182      181       -1     
- Misses         13       14       +1     

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