add 'packaging' to requirement-linting.txt to fix broken linting gate

[gaugup]

It seems like the linting gate is broken with the following error:-

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/flake8/plugins/manager.py", line [15](https://github.com/interpretml/DiCE/runs/5875796175?check_suite_focus=true#step:6:15)7, in load_plugin
    self._load()
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/flake8/plugins/manager.py", line 134, in _load
    self._plugin = self.entry_point.load()
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/importlib_metadata/__init__.py", line 203, in load
    module = import_module(match.group('module'))
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/flake8_nb/__init__.py", line 9, in <module>
    from flake8_nb.flake8_integration.formatter import IpynbFormatter
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/flake8_nb/flake8_integration/formatter.py", line 15, in <module>
    from flake8_nb.parsers.notebook_parsers import NotebookParser
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/flake8_nb/parsers/notebook_parsers.py", line [18](https://github.com/interpretml/DiCE/runs/5875796175?check_suite_focus=true#step:6:18), in <module>
    from nbconvert.filters import ipython2python
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/__init__.py", line 4, in <module>
    from .exporters import *
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/exporters/__init__.py", line 3, in <module>
    from .html import HTMLExporter
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/exporters/html.py", line 26, in <module>
    from nbconvert.filters.highlight import Highlight2HTML
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/filters/__init__.py", line 6, in <module>
    from .markdown import *
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/filters/markdown.py", line [23](https://github.com/interpretml/DiCE/runs/5875796175?check_suite_focus=true#step:6:23), in <module>
    from .pandoc import convert_pandoc
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/filters/pandoc.py", line 1, in <module>
    from nbconvert.utils.pandoc import pandoc
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/utils/pandoc.py", line 12, in <module>
    from nbconvert.utils.version import check_version
  File "/opt/hostedtoolcache/Python/3.7.12/x64/lib/python3.7/site-packages/nbconvert/utils/version.py", line 11, in <module>
    from packaging.version import Version
ModuleNotFoundError: No module named 'packaging'

The solution as per the github issue jupyter/nbconvert#1754 is to add packaging as a part of the requirement-linting.txt dependency.

[github-actions]

Unit Test Results

    1 files  ±0      1 suites  ±0   8m 18s ⏱️ - 3m 41s
242 tests ±0  237 ✔️ ±0  5 💤 ±0  0 ❌ ±0 

Results for commit 09c1e8f. ± Comparison against base commit ca7b8a8.