Update readthedocs requirements

[apcraig]

PR checklist

• Short (1 sentence) summary of your PR:
  Update readthedocs requirements to fix bibtex
• Developer(s):
  apcraig, dabail10
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  No tests carried out, this fixes an issue in the sphinx doc generation
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/. A test build of the technical docs will be performed as part of the PR testing.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:

This was actually discovered and fixed by dabail10 on a separate branch. But this needed to be added to the release branch for the documentation to build. Something changed in a recent update of sphinx (maybe a week ago), so our documentation broke at that time and this is the fix. Thanks @dabail10.