Add missing requests exceptions and warnings (#4617)

third_party/2and3/requests/exceptions.pyi

@@ -21,7 +21,13 @@ class TooManyRedirects(RequestException): ...
 class MissingSchema(RequestException, ValueError): ...
 class InvalidSchema(RequestException, ValueError): ...
 class InvalidURL(RequestException, ValueError): ...
+class InvalidHeader(RequestException, ValueError): ...
+class InvalidProxyURL(InvalidURL): ...
 class ChunkedEncodingError(RequestException): ...
 class ContentDecodingError(RequestException, BaseHTTPError): ...
 class StreamConsumedError(RequestException, TypeError): ...
 class RetryError(RequestException): ...
+class UnrewindableBodyError(RequestException): ...
+class RequestsWarning(Warning): ...
+class FileModeWarning(RequestsWarning, DeprecationWarning): ...
+class RequestsDependencyWarning(RequestsWarning): ...