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/python/input_readline_size/wsgi.py

@@ -9,7 +9,7 @@ def application(environ, start_response):
         body.append(l)
 
         if len(l) > 9:
-            body.append(b'len(l) > 9: ' + l)
+            body.append(f'len(l) > 9: {l}'.encode())
             break
 
     start_response('200', [('X-Lines-Count', str(len(body)))])