Express Python version in dependencies.yaml.

[bdice]

Description

This PR adds py_version to the dependencies.yaml file so that rapids-cmake expresses this in the same way as every other RAPIDS repository. The primary reason this might matter is for ensuring that compatible Python versions are used for style checks.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[vyasr]

/merge