Revert "build: Make Python a required dependency."

This reverts commit a2e8238b.

Reason for revert: breaks build on systems that symlink
/usr/bin/python to Python 2 rather than Python 3 -- for example,
Ubuntu 18.04.

Change-Id: Ia58fb4d9b337e65d0a7118ecbaa7e22e4b4619c2

CMakeLists.txt

@@ -30,7 +30,6 @@ file(GLOB_RECURSE
     ${PROJECT_SOURCE_DIR}/resources/*
 )
 include(FindPythonInterp)
-find_package(PythonInterp 3 REQUIRED)
 execute_process(
     COMMAND ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/gen-resources.py
     WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}