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/environment/wsgi.py

@@ -2,13 +2,10 @@ import os
 
 
 def application(env, start_response):
-    body = ''
     vars = env.get('HTTP_X_VARIABLES').split(',')
 
-    for var in vars:
-        if var in os.environ:
-            body += str(os.environ[var]) + ','
-
+    body = ','.join([str(os.environ[var]) for var in vars if var in os.environ])
     body = body.encode()
+
     start_response('200', [('Content-Length', str(len(body)))])
     return body