test: Updated asserts in test_io (#1496)

* test: Updated asserts in test_io

Updated all raw asserts to corresponding unittest.TestCase.assert. See #1483

* test: Fixed test_io

In order to use unittest.TestCase for asserts in temp_video, the object has to be passed itself to temp_video.

* test: Updated asserts to AssertionError

Switched initial raw assert to AssertionError.

* test: Changed AssertionError for ValueError

test/test_io.py

@@ -53,8 +53,10 @@ def _create_video_frames(num_frames, height, width):
 @contextlib.contextmanager
 def temp_video(num_frames, height, width, fps, lossless=False, video_codec=None, options=None):
     if lossless:
-        assert video_codec is None, "video_codec can't be specified together with lossless"
-        assert options is None, "options can't be specified together with lossless"
+        if video_codec is not None:
+            raise ValueError("video_codec can't be specified together with lossless")
+        if options is not None:
+            raise ValueError("options can't be specified together with lossless")
         video_codec = 'libx264rgb'
         options = {'crf': '0'}