[MNT] resolve dependency conflict in docs requirements

[fkiraly]

the skbase documentation currently does not build due to conflicting requirements in the docs dependency set.

The error is this:

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
sphinx-rtd-theme 1.2.0 requires docutils<0.19, but you have docutils 0.19 which is incompatible.

This PR resolves the problem by copy-pasting the dependency set from sktime.

[codecov-commenter]

Codecov Report

Merging #173 (a37a8d0) into main (c94e769) will not change coverage.
The diff coverage is n/a.

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  Coverage   82.46%   82.46%           
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  Files          39       39           
  Lines        2777     2777           
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  Hits         2290     2290           
  Misses        487      487