v0.4.0

Added

• Getters and setters for RichResponse's attributes (and deprecation warnings
  to set_*() methods).
• Getter and setter for WebhookClient's followup_event attribute (and
  deprecation warning to set_followup_event() method).
• Docs: Examples to WebhookClient's methods docstrings.
• Docs: Examples to RichResponse's attributes docstrings.
• Docs: "See also" sections in RichResponse's docstrings.
• Docs: Type hints to WebhookClient's handle_request() method's docstring.
• Docs: "Detailed example" section in "Fulfillment overview" page.

Improved

• Typing annotations coverage.