Summary of changes

Added a property fragment to URIRef so it is easy to access the URI's fragment:

>>> URIRef("http://example.com/some/path/#some-fragment").fragment
'some-fragment'
>>> URIRef("http://example.com/some/path/").fragment
''

Why is this useful?

The non-fragment part of the URI often just serves a namespace purpose, e.g. https://brickschema.org/schema/Brick#.

Actual "things" use that namespace, and have the "thing" name as the fragment, e.g. https://brickschema.org/schema/Brick#Valve_Position_Sensor

For display purposes, it's nice to get to the "thing" name, i.e. the fragment easily.

TODO: update docs and tests. I'll do that if you deem this PR is useful.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.