[sonic-config-engine] Pin 'importlib-resources' package to v3.3.1 for Python 2

[jleveque]

- Why I did it

importlib-resources v4.0.0 was released today (2020-12-23) and drops support for Python 2. This caused the sonic-config-engine Python 2 wheel build to fail.

Reference: https://pypi.org/project/importlib-resources/

Example failure:

16:23:36    File "/sonic/src/sonic-config-engine/tests/../sonic-cfggen", line 33, in <module>
16:23:36      import netaddr
16:23:36    File "/sonic/src/sonic-config-engine/.eggs/netaddr-0.8.0-py2.7.egg/netaddr/__init__.py", line 18, in <module>
16:23:36      from netaddr.core import (AddrConversionError, AddrFormatError,
16:23:36    File "/sonic/src/sonic-config-engine/.eggs/netaddr-0.8.0-py2.7.egg/netaddr/core.py", line 11, in <module>
16:23:36      from netaddr.compat import _callable, _iter_dict_keys
16:23:36    File "/sonic/src/sonic-config-engine/.eggs/netaddr-0.8.0-py2.7.egg/netaddr/compat.py", line 93, in <module>
16:23:36      import importlib_resources as _importlib_resources
16:23:36    File "/sonic/src/sonic-config-engine/.eggs/importlib_resources-4.0.0-py2.7.egg/importlib_resources/__init__.py", line 3, in <module>
16:23:36      from ._common import (
16:23:36    File "/sonic/src/sonic-config-engine/.eggs/importlib_resources-4.0.0-py2.7.egg/importlib_resources/_common.py", line 6, in <module>
16:23:36      from pathlib import Path
16:23:36  ImportError: No module named pathlib
16:23:36  ERROR

- How I did it

• Pin 'importlib-resources' package to v3.3.1 for Python 2
• Unrelated: remove pinned version of zipp for sonic-bgpcfgd because we no longer build a Python 2 version of that package.

- How to verify it

- 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)