🔧 MAINTAIN: Re-add ipykernel as dependency

[chrisjsewell]

This was unintentionally removed,
since it was previously pulled in via another dependency.

[codecov]

Codecov Report

Merging #408 (3d8e00f) into master (e17e738) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #408   +/-   ##
=======================================
  Coverage   80.44%   80.44%           
=======================================
  Files          21       21           
  Lines        2132     2132           
=======================================
  Hits         1715     1715           
  Misses        417      417           

Flag	Coverage Δ
pytests	80.44% <ø> (ø)

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