Fix typo in setup.cfg warning configuration

It appears that we were relying on the "errors" filter to treat
PendingDeprecationWarning as an error due to a typo in setup.cfg. This
line is apparently redundant with the "error" line anyway, but it's best
to have a working version of it if we're going to have it at all. I am
leaving it in place to be explicit that even these "ignored by default"
warnings should be errors, in case a default changes later.

setup.cfg

@@ -55,7 +55,7 @@ xfail_strict = true
 filterwarnings =
     error
     error::DeprecationWarning
-    error:PendingDeprecationWarning
+    error::PendingDeprecationWarning
 markers =
     gettz
     import_star