We don't need separate implementations for this class, let's not complicate it with an ABC

TypeVar is for generics, but the usage here is as an alias

The only pydantic usage was to convert strings to `Version` objects. The reason to do this conversion was to allow the register decorator to accept strings. MigrationEntry is only created inside this class, so we can just create versions from each migration when instantiating MigrationEntry instead.

Also, pydantic doesn't provide runtime time checking for arbitrary classes like Version, so we don't get any real benefit.

This was checking a `Version` object against a `MigrationEntry`, but what we want is to check the version object against `MigrationEntry.from_version`

- Remove `Migrations` class - unnecessary complexity on top of `MigrationEntry`
- Move common classes to `config_common`
- Tidy docstrings, variable names

…e migration classes

Need to sort the migrations first.

…nfig`

This prevents tests from triggering config related parsing on your "live" root.