fix date and datetime intervals from string

[hitbox]

Fix DateInterval and DateTimeInterval initialization from a string.

[AlexandreDecan]

@hitbox Can I suggest you to have a look at https://github.com/AlexandreDecan/portion ? It's a Python library providing data structure and operations for intervals (atomic and interval sets).

Apologies, it's a bit of publicity for another project, but this repository does not seem to be maintained anymore.

[hitbox]

@AlexandreDecan Thanks and I probably would use portion except I'm not using intervals directly. I was trying to use a date range type in sqlalchemy-utils with sqlite3 and the fallback to a string column does not work when coming back out of the database.

I see others posting recent issues about this and thought I'd take a stab at it.

[kvesteri]

Please fix the linting errors in tests. Otherwise this looks good