0.21.0

• Add threading.Lock() to allow responses working with threading module.
• Add urllib3 Retry mechanism. See #135
• Removed internal _cookies_from_headers function
• Now add, upsert, replace methods return registered response.
  remove method returns list of removed responses.
• Added null value support in urlencoded_params_matcher via allow_blank keyword argument
• Added strict version of decorator. Now you can apply @responses.activate(assert_all_requests_are_fired=True)
  to your function to validate that all requests were executed in the wrapped function. See #183