Unnecessary ImathConfig-<build_type>.cmake file installed

When building with python support, `-DPYTHON=ON`, the build installs an addtional cmake file, named after the lowercase CMAKE_BUILD_TYPE variable, i.e. `ImathConfig-release.cmake` for the Release build type. This file is identical with the targets file for the same build type, i.e. `ImathTargets-release.cmake`. When built without support for python, only the usual `ImathConfig{,Version}.cmake` files are correctly installed.

I'm not sure, but could this be related to
https://github.com/AcademySoftwareFoundation/Imath/blob/master/src/python/config/CMakeLists.txt#L48? If you compare this to the config/CMakeLists.txt file, the latter does reference the Targets file in the export statement, so I assume this part is unnecessary in the src/python/config/CMakeLists.txt file, as long as we don't export an extra Targets file for the Python related parts.

I noticed this on a v3.0.5 tarball, but verified it happens as well with current HEAD.

Start with src/python/config/CMakeLists.txt at the export statement around line 48, then compare it with config/CMakeLists.txt. Build with -DPYTHON=ON and inspect the installed CMake package files for the Release build type. Done means the redundant ImathConfig-.cmake file is no longer installed while the expected configuration and targets files remain.

Written by the indexing model from the issue text.