Support pydantic serialization and validation

This resolves issue #1 - The depenency is optional and the library still works fine without it.

Full Changes

• Support pydantic serialization
  • Optional dependency
• Change __repr__ to use list literal notation:
  • Now is OrderedSet([1, 2, 3]) instead of OrderedSet((1, 2, 3))
  • The str representation is still {1, 2, 3}
• Use PEP 639 license metadata