Errors reported with scikit-build when running CMake with BUILD_ZFPY enabled

[GarrettDMorrison]

Configuring CMake to build zfpy results in the following errors.

CMake Error at python/scikit-build-cmake/FindPythonExtensions.cmake:299 (list):
  list GET given empty list
Call Stack (most recent call first):
  python/CMakeLists.txt:4 (include)

CMake Error at python/scikit-build-cmake/FindPythonExtensions.cmake:303 (list):
  list GET given empty list
Call Stack (most recent call first):
  python/CMakeLists.txt:4 (include)

CMake Error at python/scikit-build-cmake/FindPythonExtensions.cmake:307 (list):
  list GET given empty list
Call Stack (most recent call first):
  python/CMakeLists.txt:4 (include)

CMake Error at python/scikit-build-cmake/FindPythonExtensions.cmake:311 (list):
  list GET given empty list
Call Stack (most recent call first):
  python/CMakeLists.txt:4 (include)

CMake Error at python/scikit-build-cmake/FindPythonExtensions.cmake:315 (list):
  list GET given empty list
Call Stack (most recent call first):
  python/CMakeLists.txt:4 (include)

CMake Error at python/scikit-build-cmake/FindPythonExtensions.cmake:320 (list):
  list GET given empty list
Call Stack (most recent call first):
  python/CMakeLists.txt:4 (include)

see also: spack/spack#41649

[GarrettDMorrison]

Fixed by 63eeefa