v6.0.1

Bugfixes

• Restored back MultiDict, CIMultiDict, MultiDictProxy, and
  CIMutiDictProxy generic type arguments; they are parameterized by value type, but the
  key type is fixed by container class.

  MultiDict[int] means MutableMultiMapping[str, int]. The key type of
  MultiDict is always str, while all str-like keys are accepted by API and
  converted to str internally.

  The same is true for CIMultiDict[int] which means MutableMultiMapping[istr, int]. str-like keys are accepted but converted to istr internally. ((#682))