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_http_header.py

@@ -107,7 +107,7 @@ class TestHTTPHeader(TestApplicationPython):
         resp = self.get(
             headers={
                 'Host': 'localhost',
-                'Custom-Header': r'(),/:;<=>?@[\]{}\t !#$%&\'*+-.^_`|~',
+                'Custom-Header': r"(),/:;<=>?@[\]{}\t !#$%&'*+-.^_`|~",
                 'Connection': 'close',
             }
         )
@@ -115,7 +115,7 @@ class TestHTTPHeader(TestApplicationPython):
         assert resp['status'] == 200, 'value chars status'
         assert (
             resp['headers']['Custom-Header']
-            == r'(),/:;<=>?@[\]{}\t !#$%&\'*+-.^_`|~'
+            == r"(),/:;<=>?@[\]{}\t !#$%&'*+-.^_`|~"
         ), 'value chars custom header'
 
     def test_http_header_value_chars_edge(self):