numpy: add a selectable source for python2 support

versions.cmake

@@ -112,7 +112,11 @@ if (WIN32 AND NOT superbuild_building_prebuilt_python AND NOT ENABLE_python3 AND
       "Prebuilt Python binaries for Windows 32 bit are not provided.")
   endif ()
 else ()
-  superbuild_set_revision(numpy
+  superbuild_set_selectable_source(numpy
+    SELECT python2
+      URL     "https://www.paraview.org/files/dependencies/numpy-1.16.4.tar.gz"
+      URL_MD5 6edf7334d04d8e8849ad058ccd3b3803
+    SELECT python3 DEFAULT
       URL     "https://www.paraview.org/files/dependencies/numpy-1.19.2.tar.gz"
       URL_MD5 471156268abd8686e39e811003726ab1)
   superbuild_set_revision(scipy