Added support for Python3 Raspberry Pi CI builds (#13612)

* Fix for RTLD_GLOBAL breakage of Pi builds, and removed Eigen version change for Pi that's no longer needed

* Fixed Pi Zero OpenBLAS build problems and tidied up directories used

* More robust checks in Pi build script

* Changed output directory for Pi CI build to fix permissions problem

* Added support for Python3 Raspberry Pi CI builds

* Tidied up comments and updated Python tool template

* Cleaned up Python include path logic