Adding dedicated test_const_name.

[rwgk]

Description

Also exercises pybind11::detail::_ backward compatibility.

See also: PR #3423

Notes:

• clang-format was applied to test_const_name.cpp
• Testing with MSVC 2015 is skipped due to bizarre errors that are not worth debugging. Completeness of the duplicated code for pybind11::detail::_ is sufficiently exercised on the many other platforms.
• _<0>() and similar only works with C++17 or higher (source code comment added). This functionality is not actually used, therefore the corresponding tests were backed out.

An easy way to disable backward compatibility is to patch include/pybind11/detail/descr.h like this:

-#ifndef _
-#define PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY
+#if defined(_) && defined(PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY)

Suggested changelog entry:

[Skylion007]

If you parameterize func, and selector_expected separately, it will implicitly do an itertools.product between them.

[rwgk]

Done. Thanks!

[rwgk]

Thank you for the reviews!

[pastasonica]

Gg