Replace deprecated pkg_resources with packaging

[mattkram]

Replaces the usage of the deprecated pkg_resources library with packaging instead.

There were two usages:

1. I've removed the version comparison, as it is not currently being used for anything. We could have used the Version class in the packaging library, but this whole warning is unnecessary.
2. Replaced the custom parsing of pypi-style requirements with packaging.requirements.Requirement. This simplifies the usage significantly and removes the DeprecationWarning that was being raised.

I've also updated the pytest settings to treat warnings as errors to allow addressing deprecation warnings earlier in the future.

Resolves: #713