Enabling PyGILState_Check() for Python >= 3.6 only.

Possibly, this explains why PyGILState_Check() cannot safely be used with Python 3.4 and 3.5:

python/cpython#10267 (comment)

include/pybind11/cast.h

@@ -1348,7 +1348,7 @@ unpacking_collector<policy> collect_arguments(Args &&...args) {
 template <typename Derived>
 template <return_value_policy policy, typename... Args>
 object object_api<Derived>::operator()(Args &&...args) const {
-#if PY_VERSION_HEX >= 0x03040000
+#if PY_VERSION_HEX >= 0x03060000
     if (!PyGILState_Check()) {
         pybind11_fail("pybind11::object_api<>::operator() PyGILState_Check() failure.");
     }