Don't use the new FindPython3 script from CMake

I don't know what it is with this one, but it's finding the wrong one. We specify through the prefix that it should search in the cura-build-environment first, but apparently it's not listening to that. As a result it's getting Python 3.6, and then stops because it can't find the headers for that version. It wouldn't need the headers in this case, but if it didn't crash here it would crash during the run time because libArcus and libSavitar have been linked to Python 3.5.

CMakeLists.txt

@@ -30,24 +30,16 @@ configure_file(${CMAKE_SOURCE_DIR}/cura.desktop.in ${CMAKE_BINARY_DIR}/cura.desk
 configure_file(cura/CuraVersion.py.in CuraVersion.py @ONLY)
 
 
-# FIXME: Remove the code for CMake <3.12 once we have switched over completely.
-# FindPython3 is a new module since CMake 3.12. It deprecates FindPythonInterp and FindPythonLibs. The FindPython3
-# module is copied from the CMake repository here so in CMake <3.12 we can still use it.
-if(${CMAKE_VERSION} VERSION_LESS 3.12)
-    # Use FindPythonInterp and FindPythonLibs for CMake <3.12
-    find_package(PythonInterp 3 REQUIRED)
-
-    set(Python3_EXECUTABLE ${PYTHON_EXECUTABLE})
-
-    set(Python3_VERSION ${PYTHON_VERSION_STRING})
-    set(Python3_VERSION_MAJOR ${PYTHON_VERSION_MAJOR})
-    set(Python3_VERSION_MINOR ${PYTHON_VERSION_MINOR})
-    set(Python3_VERSION_PATCH ${PYTHON_VERSION_PATCH})
-else()
-    # Use FindPython3 for CMake >=3.12
-    find_package(Python3 REQUIRED COMPONENTS Interpreter Development)
-endif()
+# FIXME: The new FindPython3 finds the system's Python3.6 reather than the Python3.5 that we built for Cura's environment.
+# So we're using the old method here, with FindPythonInterp for now.
+find_package(PythonInterp 3 REQUIRED)
+
+set(Python3_EXECUTABLE ${PYTHON_EXECUTABLE})
 
+set(Python3_VERSION ${PYTHON_VERSION_STRING})
+set(Python3_VERSION_MAJOR ${PYTHON_VERSION_MAJOR})
+set(Python3_VERSION_MINOR ${PYTHON_VERSION_MINOR})
+set(Python3_VERSION_PATCH ${PYTHON_VERSION_PATCH})
 
 if(NOT ${URANIUM_DIR} STREQUAL "")
     set(CMAKE_MODULE_PATH "${CMAKE_MODULE_PATH};${URANIUM_DIR}/cmake")