Use pydantic settings source for loading files

[ipmb]

This is more of the "pydantic way" to do things and restores some functionality that was broken from the BaseSettings class (see #10 and #9)

It is a backwards incompatible change. The class is now validated on instantiation. The load method has been removed and a class method from_file has been added.

• [ ] update docs
• [ ] update changelog
• [ ] make new major release