Use more portable syntax using attrs package.

Newer syntax

    attr.ib(factory=dict)

is just a syntactic sugar for

    attr.ib(default=attr.Factory(dict))

It was introduced in newest version of attrs package (18.1.0)
and doesn't work with older versions.

We should either require minimum version of attrs to be 18.1.0,
or use older (slightly more verbose) syntax.
Requiring newest version is not a good solution because
Linux distributions may have older version of attrs (17.4.0 in Fedora 28),
and requiring to build (and package)
newer version just to use newer syntactic sugar in only one test
is just too much.
It's much better to fix that test to use older syntax.

Signed-off-by: Oleg Girko <ol@infoserver.lv>

tests/server.py

@@ -22,7 +22,7 @@ class FakeChannel(object):
     wire).
     """
 
-    result = attr.ib(factory=dict)
+    result = attr.ib(default=attr.Factory(dict))
 
     @property
     def json_body(self):