Tests: switched to using f-strings.

Previously, it was necessary to support older versions of Python for
compatibility.  F-strings were released in Python 3.6.  Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.

test/test_asgi_application_unix_abstract.py

@@ -16,7 +16,7 @@ class TestASGIApplicationUnixAbstract(TestApplicationPython):
 
         addr = '\0sock'
         assert 'success' in self.conf(
-            {"unix:@" + addr[1:]: {"pass": "applications/empty"}},
+            {f"unix:@{addr[1:]}": {"pass": "applications/empty"}},
             'listeners',
         )