chore: use explicit defaulting in pyobject macros

[Skylion007]

Description

• This change makes the pyobject macro uses the equals-default instead of defining an empty ctor. The behavior should be identical to before, but the ctor is now trivial, enabling more compiler optimizations.

Suggested changelog entry:

* Explicitly default all relevant ctors for pytypes in the PYBIND11_OBJECT macros and enforce the clang-tidy checks modernize-use-equals-default in macros as well.

[Skylion007]

Sigh, this is now only a bug with the NVIDIA compilers.

[rwgk]

Sigh, this is now only a bug with the NVIDIA compilers.

I'm a bit confused TBH, IIRC there were more failures before, but I cannot figure out what changed.

Before and now, I was/am only seeing this trivial issue:

"/__w/pybind11/pybind11/include/pybind11/numpy.h", line 540: error: extra ";" ignored
      PYBIND11_OBJECT_DEFAULT(dtype, object, detail::npy_api::get().PyArrayDescr_Check_);

In theory this PR could break user projects with extra ";" & -Werror, but Google-internally I just did a (very quick) global code search for PYBIND11_OBJECT_DEFAULT and PYBIND11_OBJECT_CVT_DEFAULT: the only matches are in the pybind11 sources.

Therefore I believe this PR is safe, we just need to remove the extra ";" and add a small warning in the changelog.