Merge pull request #20611 from VadimLevin:dev/vlevin/pure-python-modules

* feat: OpenCV extension with pure Python modules

* feat: cv2 is now a Python package instead of extension module

Python package cv2 now can handle both Python and C extension modules
properly without additional "subfolders" like "_extra_py_code".

* feat: can call native function from its reimplementation in Python

modules/python/test/test_misc.py

@@ -4,6 +4,12 @@ from __future__ import print_function
 import ctypes
 from functools import partial
 from collections import namedtuple
+import sys
+
+if sys.version_info[0] < 3:
+    from collections import Sequence
+else:
+    from collections.abc import Sequence
 
 import numpy as np
 import cv2 as cv
@@ -585,6 +591,31 @@ class Arguments(NewOpenCVTests):
         self.assertEqual(ints.shape, expected_shape, "Vector of integers has wrong shape.")
 
 
+class CanUsePurePythonModuleFunction(NewOpenCVTests):
+    def test_can_get_ocv_version(self):
+        import sys
+        if sys.version_info[0] < 3:
+            raise unittest.SkipTest('Python 2.x is not supported')
+
+        self.assertEqual(cv.misc.get_ocv_version(), cv.__version__,
+                         "Can't get package version using Python misc module")
+
+    def test_native_method_can_be_patched(self):
+        import sys
+
+        if sys.version_info[0] < 3:
+            raise unittest.SkipTest('Python 2.x is not supported')
+
+        res = cv.utils.testOverwriteNativeMethod(10)
+        self.assertTrue(isinstance(res, Sequence),
+                        msg="Overwritten method should return sequence. "
+                            "Got: {} of type {}".format(res, type(res)))
+        self.assertSequenceEqual(res, (11, 10),
+                                 msg="Failed to overwrite native method")
+        res = cv.utils._native.testOverwriteNativeMethod(123)
+        self.assertEqual(res, 123, msg="Failed to call native method implementation")
+
+
 class SamplesFindFile(NewOpenCVTests):
 
     def test_ExistedFile(self):