add pytest-pretty dev dependency

[samuelcolvin]

NOTE: pytest-pretty is a package I maintain. Feel free to close this PR if you don't want the extra dependency.

Motivation and Context

I find pytest-pretty helps give a useful summary of tests, particularly when a test fails:

How Has This Been Tested?

yes

Breaking Changes

no

Types of changes

• 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 change)
• Documentation update
• test improvment

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[ihrpr]

oh, this is great!