Use __new__ instead of __init__ for some builtin classes (#4555)

Closes #4514 #2630 #2686

tests/stubtest_whitelists/py3_common.txt

@@ -412,6 +412,28 @@ zipfile.ZipExtFile.read
 zipfile.ZipExtFile.readline
 zlib.compressobj
 
+# These enums derive from (int, IntEnum) or (str, Enum). Strangely,
+# at runtime, they inherit Enum.__new__, not int.__new__ or
+# str.__new__ as the mro would dictate, which is why stubtest balks.
+enum.IntEnum.__new__
+enum.IntFlag.__new__
+http.HTTPStatus.__new__
+inspect._ParameterKind.__new__
+pstats.SortKey.__new__
+re.RegexFlag.__new__
+signal.Handlers.__new__
+signal.Sigmasks.__new__
+signal.Signals.__new__
+socket.AddressFamily.__new__
+socket.AddressInfo.__new__
+socket.MsgFlag.__new__
+socket.SocketKind.__new__
+ssl.Options.__new__
+ssl.TLSVersion.__new__
+ssl.VerifyFlags.__new__
+ssl.VerifyMode.__new__
+tkinter.EventType.__new__
+
 # ==========
 # Whitelist entries that cannot or should not be fixed
 # ==========