Log Message:
imath: update to 3.1.3.

## Version 3.1.3 (September 2, 2021)

Patch release with miscellaneous fixes

* Fix undefined access of a vector when empty
* Require sphinx 4.0.3
* Build sphinx/doxygen docs with CMake
* Use PYIMATH_OVERRIDE_PYTHON_INSTALL_DIR to specify destination python modules
* Guard `__has_attribute` for compilers that don't support it
* Cuda safety fixes
* Replace stray Imath:: with IMATH_INTERNAL_NAMESPACE::