* 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
