mark Urn.expression as a regex

[azmeuk]

It fixes this warning that is raised by pytest on projects depending on webdavclient3

    webdav3/urn.py:11: DeprecationWarning: invalid escape sequence '\.'
        expressions = "/\.+/", "/+"

[meshy]

@azmeuk thank you for this.

This is no longer a DeprecationWarning, it's a SyntaxError.

@ezhov-evgeny would it be possible to have a release that fixes this?

---

UPDATE: I discovered that I was seeing a SyntaxError because pytest was configured to turn warnings into errors.

Adding the following to my pytest.ini has got this working for me, although it will raise a SyntaxError in later versions of Python:

# This was here already:
[pytest]
filterwarnings =
    error::DeprecationWarning
    # The following is new:
    ignore:invalid escape sequence '\\.':DeprecationWarning