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

Call urllib.request.urlcleanup() to reset the global
urllib.request._opener.

Lib/test/test_site.py

@@ -525,6 +525,8 @@ class ImportSideEffectTests(unittest.TestCase):
         # 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:

Misc/NEWS.d/next/Tests/2020-10-05-09-37-43.bpo-41939.P4OlbA.rst

@@ -0,0 +1,3 @@
+Fix test_site.test_license_exists_at_url(): call
+``urllib.request.urlcleanup()`` to reset the global
+``urllib.request._opener``. Patch by Victor Stinner.