python: optional transformer argument for parameter types

[kieran-ryan]

🤔 What's changed?

• Made the Cucumber Expressions parameter type optional with the Python implementation

⚡️ What's your motivation?

• Matches the JavaScript implementation of casting to type by default if a transformer is not explicitly specified (see fix: ParameterType constructor #178)
• Improves user experience, removing a common use case to simply cast to the type - without further transformation. This improves code quality and maintainability.

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

[mpkorstanje]

Cheers!