Imath 3.2.1 got released

Imath https://github.com/AcademySoftwareFoundation/Imath/releases/tag/v3.2.1 got released

In the OpenEXR 3.4.0 documentation https://openexr.com/en/latest/install.html#install it is stated that it is recommended to build 3.4.0 with IMath 3.2

This is a soft requisite for #788 