[smart_holder] Introducing PYBIND11_INTERNALS_SH_DEF.

[rwgk]

• Replacing the approach of PR Connecting PYBIND11_INTERNALS_VERSION to PYBIND11_USE_SMART_HOLDER_AS_DEFAULT. #2939.
• For compatibility with PR Eliminate duplicate TLS keys for loader_life_support stack #3275 (by @jbms).

In hindsight, this is clearly the better approach.

[rwgk]

Hi @charlesbeattie, could you please help reviewing this small change (smart_holder branch)?
It was tested already via the helper/scratch PR #3282, GitHub CI & Google-internally.

(The CI-SH-DEF pypy3 macos-latest failure is a flake: occasional download issue.)

[charlesbeattie]

Looks good to me.

[rwgk]

Thanks @charlesbeattie!

I clicked through all 5 CI failures, they are all intermittent macos brew boost download issues. I'll ignore those, especially because this change was already tested PR #3282.