[sonic-config-engine] openconfig_acl.py: Make compliant with Python 3

[jleveque]

- Why I did it

Python 3 is more strict with __slots__. As per the documentation:

__slots__ are implemented at the class level by creating descriptors (Implementing Descriptors) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by __slots__; otherwise, the class attribute would overwrite the descriptor assignment.

This was apparently missed when making sonic-config-engine compliant with Python 3, and errors like the following would be seen:

tests/acl_loader_test.py:10: in <module>
    from acl_loader.main import *
acl_loader/main.py:8: in <module>
    import openconfig_acl
/usr/local/lib/python3.7/dist-packages/openconfig_acl.py:24: in <module>
    class yc_state_openconfig_acl__acl_state(PybindBase):
E   ValueError: '_pybind_generated_by' in __slots__ conflicts with class variable

- How I did it

Set _yang_name and _pybind_generated_by members in the constructors rather than in the class bodies.

- How to verify it

Run tests in tests/acl_loader_test.py (sonic-utilities) and ensure no errors are seen.

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

is this generated use the command on line 13?

[jleveque]

No. I did not regenerate the file, I simply fixed Python 3 errors, since we currently need this file to be compatible with both Python 2 and 3. Regenerating the file with Python 3 might break backward-compatibility. Once we remove support for Python 2 completely, we can regenerate the file using Python 3.