CIMultiDict can not be subclassed

[qwesda]

Describe the bug
CIMultiDict can not be subclassed because the type definition does not have the Py_TPFLAGS_BASETYPE flag set.

multidict/multidict/_multidict.c

Lines 1002 to 1007 in d786088

	static PyTypeObject cimultidict_type = {
	PyVarObject_HEAD_INIT(NULL, 0)
	"multidict._multidict.CIMultiDict", /* tp_name */
	sizeof(MultiDictObject), /* tp_basicsize */
	.tp_dealloc = (destructor)multidict_tp_dealloc,
	.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,

multidict/multidict/_multidict.c

Lines 1337 to 1342 in d786088

	static PyTypeObject cimultidict_proxy_type = {
	PyVarObject_HEAD_INIT(NULL, 0)
	"multidict._multidict.CIMultiDictProxy", /* tp_name */
	sizeof(MultiDictProxyObject), /* tp_basicsize */
	.tp_dealloc = (destructor)multidict_proxy_tp_dealloc,
	.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,

To Reproduce
The following code fails with TypeError: type 'multidict._multidict.CIMultiDict' is not an acceptable base type

from multidict import CIMultiDict

class Header(CIMultiDict):
    pass

Expected behavior
CIMultiDict should be subclassable like MultiDict

Your version of the Python
3.6 and 3.7

Your version of the multidict distribution
4.7.0