Add sock attribute for http.client.HTTPConnection in Python 3. (#3429)

The equivalent httplib.HTTPConnection class already include it for Python
2, despite the attribute not being documented for either version.

stdlib/3/http/client.pyi

@@ -129,6 +129,7 @@ class HTTPConnection:
     timeout: float
     host: str
     port: int
+    sock: Any
     if sys.version_info >= (3, 7):
         def __init__(
             self,