Add default for entry_points_for_group and handle edge case in URI's

[MHendricks]

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes

• If you passed the URI app/ with anytree<2.9.0 installed it would raise an IndexError: list index out of range error. This sanitizes that by removing trailing slashes.
• Adds a way to easily specify default entry points that get cast to importlib_metadata.EntryPoint when using Site.entry_points_for_group.