Reduce Minimum Required Python Version to 3.10

[GrammAcc]

I have removed use of the runtime features that were added in 3.11 or later.

We're still using typing features from later versions which is fine since type annotations aren't evaluated at runtime and we can run the type checker in a newer version with hatch.

Breaking changes:

1. Remove use of enum.StrEnum for linearmoney.data.FormatType.

• StrEnum was added in 3.11, so I have changed FormatType to be a regular enum.Enum with a custom __str__ that returns the value of the enum member instead of the name and value together.
• This introduces an internal change to the way that we lookup the format for locale data. Passing a string literal such as "standard" to the linearmoney.data.locale function's nformat argument will still work in the current implementation, but this is now an implementation detail and not an officially supported usage of the api. This can change at any time, so callers that are providing a format type to the locale function should use the actual members of the linearmoney.data.FormatType enum instead of passing in strings directly.
• The type checker will error if a string literal is provided, even if it works at runtime. I'm not sure if this would happen in the previous implementation since I never tried it, but StrEnums are supposed to behave like strings, so it's probably up to the specific type checker whether that was considered an error. It's always an error now, so this should make it easier to spot potentially flaky uses of string literals in locale definitions.