test: replace readfp() with read_file()

ConfigParser.readfp() has been deprecated since Python 3.2 and was
dropped completely in Python 3.11.

(cherry picked from commit ba4a1cd)

test/sysv-generator-test.py

@@ -82,7 +82,7 @@ def run_generator(self, expect_error=False):
                 cp = RawConfigParser(dict_type=MultiDict)
             cp.optionxform = lambda o: o  # don't lower-case option names
             with open(service) as f:
-                cp.readfp(f)
+                cp.read_file(f)
             results[os.path.basename(service)] = cp
 
         return (err, results)