Replace deprecated np.object with object

[vvolhejn]

I was getting deprecation warnings from NumPy when running tf2onnx:

/home/vaclav/venv3.8/lib/python3.8/site-packages/tf2onnx/tf_utils.py:53: DeprecationWarning: `np.object` is a deprecated alias for the builtin `object`. To silence this warning, use `object` by itself. Doing this will not modify any behavior and is safe. 
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
  if np_data.dtype == np.object:
/home/vaclav/venv3.8/lib/python3.8/site-packages/onnx/numpy_helper.py:93: DeprecationWarning: `np.object` is a deprecated alias for the builtin `object`. To silence this warning, use `object` by itself. Doing this will not modify any behavior and is safe. 
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
  if arr.dtype == np.object:
/home/vaclav/venv3.8/lib/python3.8/site-packages/tf2onnx/graph.py:585: DeprecationWarning: `np.object` is a deprecated alias for the builtin `object`. To silence this warning, use `object` by itself. Doing this will not modify any behavior and is safe. 
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
  is_bytes = np_val.dtype == np.object and len(np_val_flat) > 0 and isinstance(np_val_flat[0], bytes)

As per the linked guide, object should be used instead of np.object.

I just searched-and-replaced np.object with object across the codebase to fix this.

[hwangdeyu]

LGTM, thanks for your contribution!