feat: update conversion logic for std::vector<T> in Python bindings

`PyObject*` to `std::vector<T>` conversion logic:
- If user passed Numpy Array
  - If array is planar and T is a primitive type (doesn't require
    constructor call) that matches with the element type of array, then
    copy element one by one with the respect of the step between array
    elements. If compiler is lucky (or brave enough) copy loop can be
    vectorized.
    For classes that require constructor calls this path is not
    possible, because we can't begin an object lifetime without hacks.
  - Otherwise fall-back to general case
- Otherwise - execute the general case:
  If PyObject* corresponds to Sequence protocol - iterate over the
  sequence elements and invoke the appropriate `pyopencv_to` function.

`std::vector<T>` to `PyObject*` conversion logic:
- If `std::vector<T>` is empty - return empty tuple.
- If `T` has a corresponding `Mat` `DataType` than return
  Numpy array instance of the matching `dtype` e.g.
  `std::vector<cv::Rect>` is returned as `np.ndarray` of shape `Nx4` and
  `dtype=int`.
  This branch helps to optimize further evaluations in user code.
- Otherwise - execute the general case:
  Construct a tuple of length N = `std::vector::size` and insert
  elements one by one.

Unnecessary functions were removed and code was rearranged to allow
compiler select the appropriate conversion function specialization.

modules/python/test/test_legacy.py

@@ -20,8 +20,13 @@ class Hackathon244Tests(NewOpenCVTests):
         flag, ajpg = cv.imencode("img_q90.jpg", a, [cv.IMWRITE_JPEG_QUALITY, 90])
         self.assertEqual(flag, True)
         self.assertEqual(ajpg.dtype, np.uint8)
-        self.assertGreater(ajpg.shape[0], 1)
-        self.assertEqual(ajpg.shape[1], 1)
+        self.assertTrue(isinstance(ajpg, np.ndarray), "imencode returned buffer of wrong type: {}".format(type(ajpg)))
+        self.assertEqual(len(ajpg.shape), 1, "imencode returned buffer with wrong shape: {}".format(ajpg.shape))
+        self.assertGreaterEqual(len(ajpg), 1, "imencode length of the returned buffer should be at least 1")
+        self.assertLessEqual(
+            len(ajpg), a.size,
+            "imencode length of the returned buffer shouldn't exceed number of elements in original image"
+        )
 
     def test_projectPoints(self):
         objpt = np.float64([[1,2,3]])