Replace pkg_resources by packaging

[hagenw]

Closes #98

• Replaces the deprecated pkg_resources used in audobject.Parameter.__contains__() by its equivalent implementations in the external packaging library.
• Removes it uses in tests/conftest.py as it is no longer needed

[codecov[bot]]

Codecov Report

Merging #99 (f534a63) into main (4a99629) will not change coverage. The diff coverage is 100.0%.

Additional details and impacted files

| [Files](https://app.codecov.io/gh/audeering/audobject/pull/99?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering) | Coverage Δ | | |---|---|---| | [audobject/core/parameter.py](https://app.codecov.io/gh/audeering/audobject/pull/99?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkb2JqZWN0L2NvcmUvcGFyYW1ldGVyLnB5) | `100.0% <100.0%> (ø)` | |