url: Clean up logic in trim_url()

Python 2 doesn't like `thing is unicode`; it wants `thing == unicode`.
Of course, it just works with regular str in both py2 and py3… Oh well.

This is another find from @Exirel's overhaul of URL handling for 7.x.

As long as I was touching the line anyway, I added parentheses to make
the operator precedence explicit. Likely unnecessary, but probably
easier to read by some small measure.

sopel/modules/url.py

@@ -99,7 +99,7 @@ def trim_url(url):
 
             # clean unmatched parentheses/braces/brackets
             for (opener, closer) in [('(', ')'), ('[', ']'), ('{', '}'), ('<', '>')]:
-                if url[-1] is closer and url.count(opener) < url.count(closer):
+                if (url[-1] == closer) and (url.count(opener) < url.count(closer)):
                     url = url[:-1]
 
             return url