Update nbval requirement from >=0.9.0 to >=0.11.0

[dependabot]

Updates the requirements on nbval to permit the latest version.

Commits

• 373e9af Version number -> 0.11.0
• 9573ef0 Merge pull request #203 from computationalmodelling/pytest-collect-pathlib
• 5f56fae Install sphinx-rtd-theme for building docs
• 8248f58 Drop testing on Python 3.6
• ba129a1 RTD needs the path to pip install
• 8e3d157 Build docs with 'mambaforge-latest' Python
• 1d9f4db Try to fix readthedocs build
• d8d7c01 Remove some more compatibility code for old pytest (versions < 6)
• 3581d24 Correct parameter name for hook
• 76e052a Switch collection hook to use pathlib paths
• Additional commits viewable in compare view

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[dependabot]

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