Use right assert raises

[TomAugspurger]

Closes #20904

closes #20904

[TomAugspurger]

Ugh, had a test failure. Fixing :/

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b020891). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #20906   +/-   ##
=========================================
  Coverage          ?    91.8%           
=========================================
  Files             ?      153           
  Lines             ?    49384           
  Branches          ?        0           
=========================================
  Hits              ?    45336           
  Misses            ?     4048           
  Partials          ?        0

Flag	Coverage Δ
#multiple	90.19% <ø> (?)
#single	41.9% <ø> (?)

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[TomAugspurger]

Not immediately clear what the fix is. Skipping for now, #20907 to track.

[TomAugspurger]

2.7 and 3+ passed on Travis. Merging