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 82b218f36ce6ef910bda5af227a9fd5be613c94f)

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