Python u-string patterns

[kieran-ryan]

🤔 What's changed?

Introduced support for u-strings (explicit unicode strings e.g. u"") for Python step definitions.

⚡️ What's your motivation?

Resolves cucumber/vscode#173, allowing step definition patterns written with Python u-strings.

@given(u"I have 58 cukes in my belly")
def step_impl(context):
    ...

Although not required in Python 3, they are usable and are suggested in missing step definitions for Behave due to Python 2 compatibility.

Developer experience has been improved for Python unit tests by changing asserts to assert.strictEqual, which shows exact match failures in strings - so that they can be fixed - compared to simply stating condition was not True.

🏷️ 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.