Python clean up (onnx#3982)

* Python clean up

* Remove __future__ imports that are no-ops on Python 3. This code
  does not support Python 2 anymore so they're not needed.
* Remove some unused imports.
* Fix a couple things that trigger warnings on pytest:
    * In function_test.py, mark a string as raw so that the invalid
      escape sequence warning is not triggered.
    * In test_case.py, set `TestCase.__test__ = False` so that PyTest
      knows it's not a runnable test.
* Some other changes introduced by gen_doc.py that probably should
  have been made in an earlier commit. This just brings the genereated
  docs in sync with the source files.

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* remove extra blank lines before imports

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* gen_doc

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* Use types directly rather than strings now that Python > 3.6

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* add annotations future import

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* undo Changelog

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* fix mypy

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

* flake8 style

Signed-off-by: Gary Miguel <garymiguel@microsoft.com>

Co-authored-by: G. Ramalingam <grama@microsoft.com>

Author: garymm

docs/Operators.md

@@ -256,11 +256,6 @@ expect(node, inputs=[x], outputs=[y],
 ```python
 # SPDX-License-Identifier: Apache-2.0
 
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 import numpy as np  # type: ignore
 
 

onnx/__init__.py

@@ -1,10 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 import os
 
 from .onnx_cpp2py_export import ONNX_ML