Fix numpy dependency issue in tests

[jwodder]

Fixes #352.

Fix inspired by this comment.

Unfortunately, I do not see a way to ensure that users of dandi-cli who do not already have numpy installed do not run into this problem.

[codecov]

Codecov Report

Merging #356 (3ed64f2) into master (9168bfb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #356   +/-   ##
=======================================
  Coverage   81.92%   81.92%           
=======================================
  Files          55       55           
  Lines        5371     5371           
=======================================
  Hits         4400     4400           
  Misses        971      971           

Flag	Coverage Δ
unittests	81.92% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9168bfb...3ed64f2. Read the comment docs.

[yarikoptic]

Thank you @jwodder !!!! I expect the issue to be properly resolved "upstream" and workaround would let us keep going, thus merging

[jwodder]

@yarikoptic I'm not sure if this will be fixed upstream, as it only affects h5py 2.x, and the project is already on to 3.1.