Incompatibility with packaging>=22.0: cannot import name 'LegacyVersion' from 'packaging.version'

## Bug description



pip-audit is incompatible with latest version of `packaging`, due to [removed `LegacyVersion` ](https://github.com/pypa/packaging/blob/main/CHANGELOG.rst#220---2022-12-07)



## Reproduction steps



1. `pip install  pip-audit`

2. Check that `packaging` version is >= 22.0

3. Run `pip freeze | pip-audit --no-deps -r -` to check the currently installed packages



## Expected behavior



`pip-audit` installs a compatible version of `packaging`



## Screenshots and logs



```

Traceback (most recent call last):

  File "/usr/local/bin/pip-audit", line 5, in <module>

    from pip_audit._cli import audit

  File "/usr/local/lib/python3.9/site-packages/pip_audit/_cli.py", line 17, in <module>

    from pip_audit._audit import AuditOptions, Auditor

  File "/usr/local/lib/python3.9/site-packages/pip_audit/_audit.py", line 10, in <module>

    from pip_audit._dependency_source import DependencySource

  File "/usr/local/lib/python3.9/site-packages/pip_audit/_dependency_source/__init__.py", line 14, in <module>

    from .requirement import RequirementSource

  File "/usr/local/lib/python3.9/site-packages/pip_audit/_dependency_source/requirement.py", line 19, in <module>

    from pip_requirements_parser import InstallRequirement, InvalidRequirementLine, RequirementsFile

  File "/usr/local/lib/python3.9/site-packages/pip_requirements_parser.py", line 73, in <module>

    from packaging.version import LegacyVersion

ImportError: cannot import name 'LegacyVersion' from 'packaging.version' (/usr/local/lib/python3.9/site-packages/packaging/version.py)

```



## Platform information



* OS name and version: docker, python:3.9-alpine

* `pip-audit` version (`pip-audit -V`): 2.4.7

* Python version (`python -V` or `python3 -V`): 3.9.15

* `pip` version (`pip -V` or `pip3 -V`): 22.0.4

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