test: move color conversion tests to tests/test_plotly_utils/

Address review: _plotly_utils tests live under tests/test_plotly_utils/,
so move the file to tests/test_plotly_utils/colors/ and add the package
__init__.py to match the sibling validators/ directory.

Also drop the unused pytest import and the __main__ runner block.

Author: eeshsaxena

tests/test_plotly_utils/colors/__init__.py

@@ -1,5 +1,3 @@
-import pytest
-
 from _plotly_utils.colors import (
     find_intermediate_color,
     hex_to_rgb,
@@ -43,7 +41,3 @@ def test_find_intermediate_color_rgb_colortype():
         "rgb(0, 0, 0)", "rgb(10, 20, 30)", 0.5, colortype="rgb"
     )
     assert result == "rgb(5.0, 10.0, 15.0)"
-
-
-if __name__ == "__main__":
-    raise SystemExit(pytest.main([__file__, "-v"]))