refactor: use cast_allow_subclass for ISO parsing methods across date classes to accept str subtypes

[gtkacz]

Description

Alter ISO parsing methods to accept str subclasses. Closes #260.

Checklist

• Build runs successfully
• Type stubs updated
• Docs updated
• If docstrings were affected, check if they appear correctly in the docs as well as autocomplete

Release checklist (maintainers only)

• Version updated in pyproject.toml
• Version updated in src/whenever/__init__.py
• Version updated in changelog
• Branch merged
• Tag created and pushed
• Confirm publish job runs successfully
• Github release created

[ariebovenberg]

Thanks for the PR! Looks good at first glance. I'll probably relax subclass restrictions across the board for int/str/float before the next release