Fix documentation vulnerability

[hpesonen]

Summary:

Remove unnecessary requirements needed by rtd

Please make sure

• You have read contribution guidelines
• You have updated CHANGELOG.rst
• You have listed the copyright holder for the work you are submitting (see next section)

If your contribution adds, removes or somehow changes the functional behavior of the package, please check that

• You have included or updated all the relevant documentation, including docstrings
• You have added appropriate functional and unit tests to ensure the new features behave as expected
• You have run make lint, make docs and make test

and the proposed changes pass all unit tests (check step 6 of CONTRIBUTING.rst for details)

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• Code: BSD3 (https://opensource.org/licenses/BSD-3-Clause)
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