bpo-44647: Fix test_httpservers failing on Unicode characters in os.environ on Windows (GH-27161) (#27169)

GH-23638 introduced a new test for Accept: headers in CGI HTTP servers. This test serializes all of os.environ on the server side. For non-UTF8 locales this can fail for some Unicode characters found in environment variables. This change fixes the HTTP_ACCEPT test.
(cherry picked from commit 82b218f36c)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_httpservers.py

@@ -593,9 +593,18 @@ cgi_file6 = """\
 #!%s
 import os
 
-print("Content-type: text/plain")
+print("X-ambv: was here")
+print("Content-type: text/html")
 print()
-print(repr(os.environ))
+print("<pre>")
+for k, v in os.environ.items():
+    try:
+        k.encode('ascii')
+        v.encode('ascii')
+    except UnicodeEncodeError:
+        continue  # see: BPO-44647
+    print(f"{k}={v}")
+print("</pre>")
 """
 
 
@@ -850,8 +859,8 @@ class CGIHTTPServerTestCase(BaseTestCase):
             with self.subTest(headers):
                 res = self.request('/cgi-bin/file6.py', 'GET', headers=headers)
                 self.assertEqual(http.HTTPStatus.OK, res.status)
-                expected = f"'HTTP_ACCEPT': {expected!r}"
-                self.assertIn(expected.encode('ascii'), res.read())
+                expected = f"HTTP_ACCEPT={expected}".encode('ascii')
+                self.assertIn(expected, res.read())
 
 
 class SocketlessRequestHandler(SimpleHTTPRequestHandler):