Add constructor for HTTPError in urllib2/urllib.error (#2373)

It seems that code using HTTPError previously worked by accident
because we used to accept arbitrary keyword arguments when
instantiating BaseException, or any subclass of BaseException
(see https://github.com/python/typeshed/pull/2348).

This commit adds in the correct constructor (which also lets the
user specify the arguments in keyword-argument form).

Note: I'm not very familiar with the urllib libraries, so I opted
to just add the signature and leave it up to somebody else to
fill in the types.

stdlib/2/urllib2.pyi

@@ -12,6 +12,7 @@ class URLError(IOError):
 class HTTPError(URLError, addinfourl):
     code = ...  # type: int
     headers = ...  # type: Dict[str, str]
+    def __init__(self, url, code, msg, hdrs, fp) -> None: ...
 
 class Request(object):
     host = ...  # type: str

stdlib/3/urllib/error.pyi

@@ -8,4 +8,5 @@ class URLError(IOError):
 class HTTPError(URLError, addinfourl):
     code = ...  # type: int
     headers = ...  # type: Dict[str, str]
+    def __init__(self, url, code, msg, hdrs, fp) -> None: ...
 class ContentTooShortError(URLError): ...