Removing __INTEL_COMPILER section from pragma block at the top of pybind11.h (#3135)

* Fixing `pragma warning pop` for `__INTEL_COMPILER`.

* Adding push/pop to 3 tests. Removing #878 from top of pybind11.h (it was/is only needed for 1 test).

* Trying again after CI failure, moving the push to the top of 2 tests.

* Trying more after CI failure, adding push/pop to pybind11_tests.h, constructor_stats.h.

* Moving ICC #2196 suppression to CMakeLists.txt

* Fixing condition for `pragma GCC diagnostic push` in pybind11.h

* Moving `pragma warning disable 2196` to common.h

* Revising #ifdef to be more conservative.

* Undoing insertion of notes that will hopefully soon be completely obsolete anyway.