[BUG] pybind11 fails to distinguish template instantiations with auto template parameter

Issue description

pybind11 fails to distinguish template <auto D> class ... instantiations, and report

ImportError: generic_type: type "TestClass6" is already registered!

using template <size_t D> class ... works without issue.

Tested on MSVC 16.9.1, Python 3.8.3, pybind11 2.6.2

If it's unavoidable, is there any workaround?

Reproducible example code

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>

template <auto D>
class TestClass {
public:
    void test(std::array<double, D>& a) {
        a[0] = 1;
    }
};
PYBIND11_MODULE(vtc, m) {
    m.doc() = "Vehicle tracking control Python binding";

    using TestClass6 = TestClass<6>;
    py::class_<TestClass6>(m, "TestClass6")
        .def("test", &TestClass6::test);

    using TestClass8 = TestClass<8>;
    py::class_<TestClass8>(m, "TestClass8")
        .def("test", &TestClass8::test);
}

Start by building and running the reproducible module shown in the issue with the two auto-parameter instantiations, then inspect the py::class_ registration path included from pybind11/pybind11.h. Compare how TestClass6 and TestClass8 are identified during registration with the size_t version. Done means both classes import without a duplicate-registration error.