Limiting pragma for ignoring GCC 7 -Wnoexcept-type to the scope of pybind11.h. (#3161)

* Moving pragma for ignoring -Wnoexcept-type to the one location where it is needed.

* Trying a second location.

* The previous commit worked (GitHub Actions green), but see the added comment about the dicy nature of -Wnoexcept-type ("if and only if").

* Applying reviewer suggestion.