…x>` everywhere.

From PR pybind#4316 we know that types in the unnamed namespace in different translation units do not compare equal, as desired.

But do types in named namespaces compare equal, as desired?

…ype_index>` everywhere."

This reverts commit a06949a.

…ndex` only under macOS, based on results shown under pybind#4316 (comment)

…pes defined in the unnamed namespace.

…e_a & test_unnamed_namespace_b need to work when imported in any order.

…ternal testing.

Added block transferred from PR pybind#4321

Already tested under PR pybind#4321

Added block transferred from PR pybind#4324

…hanges were merged under pybind#4323 and pybind#4326)

…RSION=10000000

… windows

Extra work added to quick jobs, based on timings below, to not increase the GHA start-to-last-job-finished time.

```
Duration
^              Number of pytest runs
^              ^ Job identifier
^              ^ ^
0:03:48.024227 1 1___3___Clang_3.6___C++11___x64.txt
0:03:58.992814 1 2___3___Clang_3.7___C++11___x64.txt
0:04:25.758942 1 1___3.7___Debian___x86____Install.txt
0:04:50.148276 1 4___3___Clang_7___C++11___x64.txt
0:04:55.784558 1 13___3___Clang_15___C++20___x64.txt
0:04:57.048754 1 6___3___Clang_dev___C++11___x64.txt
0:05:00.485181 1 7___3___Clang_5___C++14___x64.txt
0:05:03.744964 1 2___3___almalinux8___x64.txt
0:05:06.222752 1 5___3___Clang_9___C++11___x64.txt
0:05:11.767022 1 2___3___GCC_7___C++17__x64.txt
0:05:18.634930 1 2___3.11__deadsnakes____x64.txt
0:05:22.810995 1 1___3___GCC_7___C++11__x64.txt
0:05:25.275317 1 12___3___Clang_14___C++20___x64.txt
0:05:32.058174 1 5___3___GCC_10___C++17__x64.txt
0:05:39.381351 1 7___3___GCC_12___C++20__x64.txt
0:05:40.502252 1 8___3___Clang_10___C++17___x64.txt
0:05:59.344905 1 3___3___Clang_3.9___C++11___x64.txt
0:06:10.825147 1 6___3___GCC_11___C++20__x64.txt
0:06:20.655443 1 3___3___almalinux9___x64.txt
0:06:22.472061 1 3___3___GCC_8___C++14__x64.txt
0:06:42.647406 1 11___3___Clang_13___C++20___x64.txt
0:06:53.352720 1 1___3.10___CUDA_11.7___Ubuntu_22.04.txt
0:07:07.357801 1 2___3.7___MSVC_2019___x86_-DCMAKE_CXX_STANDARD=14.txt
0:07:09.057603 1 1___3___centos7___x64.txt
0:07:15.546282 1 1___3.8___MSVC_2019__Debug____x86_-DCMAKE_CXX_STANDARD=17.txt
0:07:22.566022 1 4___3___GCC_8___C++17__x64.txt
0:08:13.592674 1 2___3.9___MSVC_2019__Debug____x86_-DCMAKE_CXX_STANDARD=20.txt
0:08:16.422768 1 9___3___Clang_11___C++20___x64.txt
0:08:21.168457 1 3___3.8___MSVC_2019___x86_-DCMAKE_CXX_STANDARD=17.txt
0:08:27.129468 1 10___3___Clang_12___C++20___x64.txt
0:09:35.045470 1 1___3.10___windows-latest___clang-latest.txt
0:09:57.361843 1 1___3.9___MSVC_2022_C++20___x64.txt
0:10:35.187767 1 1___3.6___MSVC_2019___x86.txt
0:11:14.691200 4 2___3.9___ubuntu-20.04___x64.txt
0:11:37.701167 1 1_macos-latest___brew_install_llvm.txt
0:11:38.688299 4 4___3.11___ubuntu-20.04___x64.txt
0:11:52.720216 1 4___3.9___MSVC_2019___x86_-DCMAKE_CXX_STANDARD=20.txt
0:13:23.456591 4 6___pypy-3.8___ubuntu-20.04___x64_-DPYBIND11_FINDPYTHON=ON.txt
0:13:25.863592 2 1___3___ICC_latest___x64.txt
0:13:32.411758 3 9___3.9___windows-2022___x64.txt
0:13:45.473377 4 3___3.10___ubuntu-20.04___x64.txt
0:13:55.366447 4 5___pypy-3.7___ubuntu-20.04___x64.txt
0:13:57.969502 3 10___3.10___windows-2022___x64.txt
0:14:19.837475 3 11___3.11___windows-2022___x64.txt
0:14:33.316770 4 1___3.6___ubuntu-20.04___x64_-DPYBIND11_FINDPYTHON=ON_-DCMA.txt
0:15:34.449278 4 22___3.6___windows-2019___x64_-DPYBIND11_FINDPYTHON=ON.txt
0:16:25.189055 2 1___3.9-dbg__deadsnakes____Valgrind___x64.txt
0:17:20.956667 4 15___3.6___macos-latest___x64.txt
0:17:27.513891 4 23___3.9___windows-2019___x64.txt
0:17:58.783286 3 8___3.6___windows-2022___x64.txt
0:18:25.917828 4 7___pypy-3.9___ubuntu-20.04___x64.txt
0:19:17.399820 3 13___pypy-3.8___windows-2022___x64.txt
0:19:45.002122 3 12___pypy-3.7___windows-2022___x64.txt
0:20:03.201926 4 16___3.9___macos-latest___x64.txt
0:20:15.415178 4 17___3.10___macos-latest___x64.txt
0:20:20.263216 4 20___pypy-3.8___macos-latest___x64.txt
0:20:31.998226 3 1___3___windows-latest___mingw64.txt
0:20:40.812286 4 18___3.11___macos-latest___x64.txt
0:22:47.714749 4 19___pypy-3.7___macos-latest___x64.txt
0:23:04.435859 3 2___3___windows-latest___mingw32.txt
0:25:48.719597 3 14___pypy-3.9___windows-2022___x64.txt
0:26:01.211688 4 21___pypy-3.9___macos-latest___x64.txt
0:28:19.971015 1 1___3___CentOS7__PGI_22.9___x64.txt
```

…arate PR).