Fix check_contracts handling of forward references in type annotations

[david-yz-liu]

Motivation and Context

A recent change caused @check_contracts to evaluate instance attribute type annotations when the class definition was first reached, which raised an error when the type annotations had forward references. One example is for linked list nodes:

@check_contracts
class Node:
    item: Any
    next: Optional[Node]

Your Changes

Description: Delay evaluate of the class type annotations until the first time an attribute is set (or a method is called).

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)

Testing

Added test cases and manual testing.

Questions and Comments (if applicable)

Checklist

• I have performed a self-review of my own code.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have added tests for my changes.
• I have updated the CHANGELOG.md file.