Drop pydantic = 1 support?

[fjclark]

Pydantic 1 support was added in #114 though I don't know what the use-case was. However, I'm not clear on whether this is still required, and it would be cleaner to drop support if it isn't (for example, this complicates testing and environment set up slightly, e.g. #129 (comment)).

@SimonBoothroyd what was the original use-case and would you suggest keeping support? Thanks.

[lilyminium]

The main thing OpenFF-wise is that this would allow us to have bespokefit and current qcarchive packages in the same environment, I believe... but the rest of the ecosystem is moving on, and MolSSI is working on rolling out Pydantic 2 support. @SimonBoothroyd just pinging again if there was anything you had in mind?