Fix importlib_metadata incompatibility

[rly]

Motivation

Gallery tests are failing because pip install requirements-doc.txt after installing the latest hdmf results in an incompatibility in the requirements for python 3.7. This PR fixes it.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1694 (372dc37) into dev (2eb2ddf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1694   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files          26       26           
  Lines        2592     2592           
  Branches      679      679           
=======================================
  Hits         2383     2383           
  Misses        134      134           
  Partials       75       75           

Flag	Coverage Δ
integration	71.52% <ø> (ø)
unit	83.56% <ø> (ø)

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