Generalize return types from classmethods

[gadomski]

Related Issue(s):

• Closes alternative constructors shouldn't hardcode their return types #862
• Closes clone shouldn't hard-code return type #863

Description:

Use TypeVar for alternate constructors, e.g. from_dict and from_file. The key cast that enables the whole inheritance tree to work is https://github.com/stac-utils/pystac/compare/issues/862-return-types?expand=1#diff-3f0f0b208e42045d63bb010a3b963e4178a8cf882f75f73d8392828b2190eafcR525.

Includes some sidecar fixes:

• Allow subclass instantiation in clone in a couple of places where it was missing, closing clone shouldn't hard-code return type #863
• Fix up a couple more from __future__ import annotations opportunities that were missed in Use from __future__ import annotations #962
• Begin using simple test fixtures, now that we have pytest from Switch to pytest #939
• Fix a couple of lints that I flagged locally with ruff

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[pjhartzell]

Looks good. Just a few questions.