Fix handling of read h5py string datasets

[rly]

Motivation

Fix #1186.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.01%. Comparing base (044f5a5) to head (9ecdf4f).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1189   +/-   ##
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  Coverage   89.01%   89.01%           
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  Files          45       45           
  Lines        9872     9872           
  Branches     2810     2810           
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  Hits         8788     8788           
  Misses        767      767           
  Partials      317      317           

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[stephprince]

Looks good. For the python 3.8 test failures, maybe the string dataset needs to be created differently since that environment uses h5py 2.10?