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_python_isolation_chroot.py

@@ -16,7 +16,7 @@ class TestPythonIsolation(TestApplicationPython):
         self.load('ns_inspect', isolation=isolation)
 
         assert (
-            self.getjson(url='/?path=' + temp_dir)['body']['FileExists']
+            self.getjson(url=f'/?path={temp_dir}')['body']['FileExists']
             == False
         ), 'temp_dir does not exists in rootfs'