Skip cython test if it does not exist.

test cases/python3/3 cython/meson.build

@@ -1,7 +1,9 @@
 project('cython', 'c',
   default_options : ['warning_level=3'])
 
-cython = find_program('cython3')
+cython = find_program('cython3', required : false)
+
+if cython.found()
   py3_dep = dependency('python3')
 
   subdir('libdir')
@@ -10,3 +12,6 @@ test('cython tester',
     find_program('cytest.py'),
     env : ['PYTHONPATH=' + pydir]
   )
+else
+  message('Cython not found, skipping test.')
+endif