Fix doctest for widen

[kimikage]

My stance is that doctest is a tool for finding errors in documentation, and there is no need to run it on anything other than the latest release (or LTS).
IIRC, in a discussion somewhere other than this package, I received a counterargument along the lines of "It’s a good thing that doctest passes."

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.22%. Comparing base (a26b6d8) to head (92b56dc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #357   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files           7        7           
  Lines         792      792           
=======================================
  Hits          770      770           
  Misses         22       22           

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[kimikage]

We will review the issue found in the nightly build before v0.9.3 release.