getUrls method for single CSSRule, not CSSStyleSheet?

currently `getUrls` method could be applied to `CSSStyleSheet` object only. Is there a simple way to get urls from single CSS rule?
My workaround is like following:

```python
from cssutils import CSSParser
from cssutils.css import CSSStyleRule
from cssutils.css import CSSStyleSheet
from cssutils import getUrls

parser = CSSParser()
style = parser.parseStyle(cssText="height:450px; background-image: url('https://example.com/image.png');")

rule = CSSStyleRule(selectorText="html", style=style)

css = CSSStyleSheet()
css.add(rule)
print(list(getUrls(css)))
```

but it seems to be a bit over-engineered. Is there a better/faster way?

Start by reading the getUrls entry point and the CSSRule and CSSStyleSheet APIs mentioned in the issue. Check how getUrls traverses a stylesheet, then determine whether the same URL extraction can support a single CSSRule directly. Done means retrieving the URL from the provided CSSStyleRule without wrapping it in a CSSStyleSheet, with coverage for the demonstrated style.

Written by the indexing model from the issue text.