Skip cython test if it does not exist.

test cases/python3/3 cython/meson.build

@@ -1,12 +1,17 @@
 project('cython', 'c',
   default_options : ['warning_level=3'])
 
-cython = find_program('cython3')
-py3_dep = dependency('python3')
+cython = find_program('cython3', required : false)
 
-subdir('libdir')
+if cython.found()
+  py3_dep = dependency('python3')
 
-test('cython tester',
-  find_program('cytest.py'),
-  env : ['PYTHONPATH=' + pydir]
-)
+  subdir('libdir')
+
+  test('cython tester',
+    find_program('cytest.py'),
+    env : ['PYTHONPATH=' + pydir]
+  )
+else
+  message('Cython not found, skipping test.')
+endif