Eliminate duplicate TLS keys for loader_life_support stack (#3275)

* Eliminate duplicate TLS keys for loader_life_support stack

This revises the existing fix for
https://github.com/pybind/pybind11/issues/2765 in
https://github.com/pybind/pybind11/pull/3237 to reduce the amount of
TLS storage used.

The shared TLS key is stored in two different ways, depending on
`PYBIND11_INTERNALS_VERSION`.  If `PYBIND11_INTERNALS_VERSION ==
4` (as is currently set), the TLS key is stored in the
`internal::shared_data` map to avoid breaking ABI compatibility.  If
`PYBIND11_INTERNALS_VERSION > 4`, the TLS key is stored directly in
the `internals` struct.

* Fix test_pytypes.py::test_issue2361 failure on PyPy3.7

* Add github actions tests for unstable ABI

.github/workflows/ci.yml

@@ -144,6 +144,24 @@ jobs:
       if: "!(runner.os == 'Windows' && (matrix.python == 3.8 || matrix.python == 3.9 || matrix.python == '3.10-dev'))"
       run: cmake --build build2 --target cpptest
 
+    # Third build - C++17 mode with unstable ABI
+    - name: Configure (unstable ABI)
+      run: >
+        cmake -S . -B build3
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+        -DPYBIND11_INTERNALS_VERSION=10000000
+        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+        ${{ matrix.args }}
+
+    - name: Build (unstable ABI)
+      run: cmake --build build3 -j 2
+
+    - name: Python tests (unstable ABI)
+      run: cmake --build build3 --target pytest
+
     - name: Interface test
       run: cmake --build build2 --target test_cmake_build