added backwards compatibility with opencv 4.5.2 and below (keypoints) (#139)

* Update draw.py 

Error in draw 
TypeError: KeyPoint() missing required argument '_size' (pos 3)

was size=3 
change to _size=3

* handle get_cv_keypoints

wraps cv2.KeyPoint(..., size=3), cv2.KeyPoint(..., _size=3) in try except block to handle change in argument name between opencv versions.

* Update tf_bodypix/draw.py

Co-authored-by: Daniel Ecer <de-code@users.noreply.github.com>

* Update tf_bodypix/draw.py

Co-authored-by: Daniel Ecer <de-code@users.noreply.github.com>

Co-authored-by: Daniel Ecer <de-code@users.noreply.github.com>

Author: Tenfleques

tf_bodypix/draw.py

@@ -41,14 +41,25 @@ def get_adjacent_keypoints(
 
 
 def get_cv_keypoints(keypoints: Iterable[Keypoint]) -> List[cv2.KeyPoint]:
-    return [
-        cv2.KeyPoint(
-            x=keypoint.position.x,
-            y=keypoint.position.y,
-            size=3
-        )
-        for keypoint in keypoints
-    ]
+    try:
+        return [
+            cv2.KeyPoint(
+                x=keypoint.position.x,
+                y=keypoint.position.y,
+                size=3
+            )
+            for keypoint in keypoints
+        ]
+    except TypeError:
+        # backwards compatibility with opencv 4.5.2 and below
+        return [
+            cv2.KeyPoint(
+                x=keypoint.position.x,
+                y=keypoint.position.y,
+                _size=3
+            )
+            for keypoint in keypoints
+        ]
 
 
 def draw_skeleton(