fix circular reference for html repr

[bendichter]

Motivation

fix #1057

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0998190) 88.53% compared to head (ecd56e1) 88.51%.

Files	Patch %	Lines
src/hdmf/container.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1058      +/-   ##
==========================================
- Coverage   88.53%   88.51%   -0.02%     
==========================================
  Files          45       45              
  Lines        9605     9607       +2     
  Branches     2732     2733       +1     
==========================================
  Hits         8504     8504              
- Misses        778      779       +1     
- Partials      323      324       +1     

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