docs(exceptions): Remove unused exception URLRequired from documentation The documentation previously listed `requests.URLRequired` as a valid exception, suggesting it would be raised for invalid URLs. However, this exception has been dead code since commit ab27027 (2012) and is never actually raised. Instead, invalid URLs raise `MissingSchema`, `InvalidSchema`, or `InvalidURL`, none of which were documented. This commit removes `URLRequired` from the documentation to reflect the actual behavior and prevent confusion. Signed-off-by: jakobheine <me@jakobheine.de>