urllib2: Use addbase to model readable IO

<https://docs.python.org/2/library/urllib2.html#urllib2.urlopen>
> urllib2.urlopen(url[,data[,timeout[,cafile[,capath[,cadefault[,context]]]]])
...
> This function returns a file-like object with three additional
> methods:
> - geturl()
> - info()
> - getcode()
...
> Note that None may be returned if no handler handles the request.

First suggested by srittau at
<#2688 (review)>
but then changed to just use `class addbase` inspired by commit
1442cc0 from Michael Brandt.

stdlib/2/urllib2.pyi

@@ -12,7 +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: ...
+    def __init__(self, url, code, msg, hdrs, fp: addinfourl) -> None: ...
 
 class Request(object):
     host = ...  # type: str
@@ -49,12 +49,12 @@ class OpenerDirector(object):
     addheaders: List[Tuple[str, str]]
 
     def add_handler(self, handler: BaseHandler) -> None: ...
-    def open(self, fullurl: Union[Request, _string], data: Optional[_string] = ..., timeout: Optional[float] = ...): ...
+    def open(self, fullurl: Union[Request, _string], data: Optional[_string] = ..., timeout: Optional[float] = ...) -> Optional[addinfourl]: ...
     def error(self, proto: _string, *args: Any): ...
 
 def urlopen(url: Union[Request, _string], data: Optional[_string] = ..., timeout: Optional[float] = ...,
             cafile: Optional[_string] = ..., capath: Optional[_string] = ..., cadefault: bool = ...,
-            context: Optional[ssl.SSLContext] = ...): ...
+            context: Optional[ssl.SSLContext] = ...) -> Optional[addinfourl]: ...
 def install_opener(opener: OpenerDirector) -> None: ...
 def build_opener(*handlers: Union[BaseHandler, Type[BaseHandler]]) -> OpenerDirector: ...
 
@@ -70,16 +70,16 @@ class HTTPErrorProcessor(BaseHandler):
     def http_response(self, request, response): ...
 
 class HTTPDefaultErrorHandler(BaseHandler):
-    def http_error_default(self, req: Request, fp, code, msg, hdrs): ...
+    def http_error_default(self, req: Request, fp: addinfourl, code, msg, hdrs): ...
 
 class HTTPRedirectHandler(BaseHandler):
     max_repeats = ...  # type: int
     max_redirections = ...  # type: int
-    def redirect_request(self, req: Request, fp, code, msg, headers, newurl): ...
-    def http_error_301(self, req: Request, fp, code, msg, headers): ...
-    def http_error_302(self, req: Request, fp, code, msg, headers): ...
-    def http_error_303(self, req: Request, fp, code, msg, headers): ...
-    def http_error_307(self, req: Request, fp, code, msg, headers): ...
+    def redirect_request(self, req: Request, fp: addinfourl, code, msg, headers, newurl): ...
+    def http_error_301(self, req: Request, fp: addinfourl, code, msg, headers): ...
+    def http_error_302(self, req: Request, fp: addinfourl, code, msg, headers): ...
+    def http_error_303(self, req: Request, fp: addinfourl, code, msg, headers): ...
+    def http_error_307(self, req: Request, fp: addinfourl, code, msg, headers): ...
     inf_msg = ...  # type: str
 
 
@@ -106,11 +106,11 @@ class AbstractBasicAuthHandler:
 
 class HTTPBasicAuthHandler(AbstractBasicAuthHandler, BaseHandler):
     auth_header = ...  # type: str
-    def http_error_401(self, req: Request, fp, code, msg, headers): ...
+    def http_error_401(self, req: Request, fp: addinfourl, code, msg, headers): ...
 
 class ProxyBasicAuthHandler(AbstractBasicAuthHandler, BaseHandler):
     auth_header = ...  # type: str
-    def http_error_407(self, req: Request, fp, code, msg, headers): ...
+    def http_error_407(self, req: Request, fp: addinfourl, code, msg, headers): ...
 
 class AbstractDigestAuthHandler:
     def __init__(self, passwd: Optional[HTTPPasswordMgr] = ...) -> None: ...
@@ -127,12 +127,12 @@ class AbstractDigestAuthHandler:
 class HTTPDigestAuthHandler(BaseHandler, AbstractDigestAuthHandler):
     auth_header = ...  # type: str
     handler_order = ...  # type: int
-    def http_error_401(self, req: Request, fp, code, msg, headers): ...
+    def http_error_401(self, req: Request, fp: addinfourl, code, msg, headers): ...
 
 class ProxyDigestAuthHandler(BaseHandler, AbstractDigestAuthHandler):
     auth_header = ...  # type: str
     handler_order = ...  # type: int
-    def http_error_407(self, req: Request, fp, code, msg, headers): ...
+    def http_error_407(self, req: Request, fp: addinfourl, code, msg, headers): ...
 
 class AbstractHTTPHandler(BaseHandler):  # undocumented
     def __init__(self, debuglevel: int = ...) -> None: ...