Add statsmodels package to requirements.txt to pin readthedocs build

[elehmer]

Description of PR

ReadtheDocs build is failing on statsmodels requirement from xclim==0.42.0. For some reason it is trying older and older versions to find compatiable dask_expr package. Attempting to pin statsmodels==0.14.1 which was last successful build version.

Summary of changes and related issue

Relevant motivation and context

Dependencies required for this change?

Fixes # (issue), delete if not necessary

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

• [ ] Test A
• [ ] Test B

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules