bpo-41939: Fix test_site.test_license_exists_at_url() (#22559)

Call urllib.request.urlcleanup() to reset the global
urllib.request._opener.
diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index d3ee68f..a475ed1 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -525,6 +525,8 @@
         # string displayed by license in the absence of a LICENSE file.
         url = license._Printer__data.split()[1]
         req = urllib.request.Request(url, method='HEAD')
+        # Reset global urllib.request._opener
+        self.addCleanup(urllib.request.urlcleanup)
         try:
             with socket_helper.transient_internet(url):
                 with urllib.request.urlopen(req) as data: