Fix appveyor

[freddyaboulton]

Reference Issues/PRs

Fixes #1539

What does this implement/fix? Explain your changes.

Does your contribution introduce a new dependency? If yes, which one?

What should a reviewer concentrate their feedback on?

Any other comments?

PR checklist

For all contributions

• I've added myself to the list of contributors.
• Optionally, I've updated sktime's CODEOWNERS to receive notifications about future changes to these files.
• I've added unit tests and made sure they pass locally.

For new estimators

• I've added the estimator to the online documentation.
• I've updated the existing example notebooks or provided a new one to showcase how my estimator works.

[fkiraly]

this ... seems to have worked? 😮

[freddyaboulton]

@fkiraly I believe so! Will open it up for review now.

[fkiraly]

and only 12 characters change... really an odd story.

[mloning]

Great stuff, thanks @freddyaboulton

[fkiraly]

"Once more, @freddyaboulton had saved the day."