pin sphinx < 6.0

[sappelhoff]

PR Description

fixes current circleci issues ... but I have no clue about what's the actual problem.

See here: https://app.circleci.com/pipelines/github/mne-tools/mne-bids/4980/workflows/2e885df3-ebb4-4a46-9b83-16decafba847/jobs/7129

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #1115 (058f53e) into main (0c83b75) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1115   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files          40       40           
  Lines        8670     8670           
=======================================
  Hits         8240     8240           
  Misses        430      430           

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[drammock]

I have no clue about what's the actual problem

sphinx 6.0 removed built-in variable "logo" in favor of "logo_url". It's being fixed at the theme level (in-progress) pydata/pydata-sphinx-theme#1097