[defaults] Upgrade to python 3.7 exact.

We force python version to be exactly 3.7 as it is the only one tested.
USE_SYSTEM_python3 is used to define if python libs are directly linked
to the python3 system directory (USE_SYSTEM_python3=ON) or shipped
in the install dir (OFF) (See superbuild_python_install function).

CMakeLists.txt

@@ -69,7 +69,7 @@ function (superbuild_find_projects var)
     cxx11
     paraview
     python
-    python2
+    python3
     qt5
     pythonqt
     boost
@@ -130,29 +130,30 @@ if (WIN32)
 endif()
 
 if(UNIX)
-  if(NOT DEFINED USE_SYSTEM_python2)
-    # it is still possible to pass -DUSE_SYSTEM_python2=False to cmake
-    message(STATUS "Defaulting USE_SYSTEM_python2 to True (was undefined and we are on UNIX)")
-    set(USE_SYSTEM_python2 True CACHE BOOL INTERNAL)
+  # The following variable is used to define if python libs have to be used from the system 
+  # directory (USE_SYSTEM_python3=True) or shipped in install dir (USE_SYSTEM_python3=False)
+  # this behavior is managed by the superbuild python install functions
+  if(NOT DEFINED USE_SYSTEM_python3)
+    # it is still possible to pass -DUSE_SYSTEM_python3=False to cmake
+    message(STATUS "Defaulting USE_SYSTEM_python3 to True (was undefined and we are on UNIX)")
+    set(USE_SYSTEM_python3 True CACHE BOOL INTERNAL)
   endif()
 
-  if(USE_SYSTEM_python2)
-    # This is needed to ensure the superbuild projects to use the same python version.  
-    # (Only needed when a python system is required)
-    find_package(Python2 2.7 QUIET REQUIRED COMPONENTS Interpreter Development)
+  # This is needed to ensure the superbuild projects to use the same python version.
+  # We force python to version 3.7 as it is the only one that has been tested
+  find_package(Python3 3.7 EXACT QUIET REQUIRED COMPONENTS Interpreter Development)
 
-    # By setting the following variables we specify a python version for FindPythonLibs
-    # (See cmake FindPythonLibs documentation)
-    set(PYTHON_INCLUDE_DIRS ${Python2_INCLUDE_DIRS})
-    set(PYTHON_INCLUDE_DIR ${Python2_INCLUDE_DIRS})
-    set(PYTHON_LIBRARY ${Python2_LIBRARIES})
-    
-    # Python2_INCLUDE_DIR and Python2_LIBRARY are Artefacts in FindPython2
-    # Both can be used to specific a python version
-    # (See cmake FindPython2 documentations)
-    set(Python2_INCLUDE_DIR ${Python2_INCLUDE_DIRS})
-    set(Python2_LIBRARY ${Python2_LIBRARIES})
-  endif()
+  # By setting the following variables we specify a python version for FindPythonLibs
+  # (See cmake FindPythonLibs documentation)
+  set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
+  set(PYTHON_INCLUDE_DIR ${Python3_INCLUDE_DIRS})
+  set(PYTHON_LIBRARY ${Python3_LIBRARIES})
+  
+  # Python3_INCLUDE_DIR and Python3_LIBRARY are Artefacts in FindPython3
+  # Both can be used to specific a python version
+  # (See cmake FindPython3 documentations)
+  set(Python3_INCLUDE_DIR ${Python3_INCLUDE_DIRS})
+  set(Python3_LIBRARY ${Python3_LIBRARIES})
 endif()
 
 function (superbuild_add_packaging)

Projects/paraview.cmake

@@ -6,7 +6,7 @@ if (Protobuf_FOUND)
   list(APPEND pv_cmake_options "-DVTK_USE_SYSTEM_PROTOBUF:BOOL=${VTK_USE_SYSTEM_PROTOBUF}" )
 endif()
 superbuild_add_project(paraview
-  DEPENDS qt5 python python2 pythonqt
+  DEPENDS qt5 python python3 pythonqt
   DEFAULT_ON
   DEBUGGABLE
   CMAKE_ARGS

Projects/pythonqt.cmake

 superbuild_add_project(pythonqt
-  DEPENDS qt5 python python2
+  DEPENDS qt5 python python3
   CMAKE_ARGS
     -DBUILD_SHARED_LIBS:BOOL=ON
     -DPythonQt_QT_VERSION:STRING=${qt_version}