For 3.9.0, test_site fails with an env changed warning:

Warning -- urllib.requests._opener was modified by test_site
  Before: None
  After:  <urllib.request.OpenerDirector object at 0xb76f4f88>
test_site failed (env changed)

== Tests result: SUCCESS ==

1 test altered the execution environment:
    test_site

The problem occurs in test_site's test_license_exists_at_url test and was introduced by the changes in Issue37421: "Some tests leak temporary files".

This failure has not been noticed up to now because test_license_exists_at_url is skipped unless the build being tested is for a released branch, which is about to happen for 3.9 with the 3.9.0 release.  The test failure can be reproduced on any build by commenting out Lib/test_site.py lines 503-504:

    @unittest.skipUnless(sys.version_info[3] == 'final',
                         'only for released versions')

Setting to "deferred blocker" status for release manager evaluation.

It's a minor issue in the test itself, not in urllib. Such bug should not block a release.

I proposed PR 22559 to fix the issue.

See also bpo-37475: "What is urllib.request.urlcleanup() function?".

New changeset 1fce240d6c4b2b2cc17a86e88c65169e15b9feeb by Victor Stinner in branch 'master':
bpo-41939: Fix test_site.test_license_exists_at_url() (#22559)
https://github.com/python/cpython/commit/1fce240d6c4b2b2cc17a86e88c65169e15b9feeb

New changeset 10b4136bfa5878c058753e1d1bd091e0f7e1ba40 by Miss Skeleton (bot) in branch '3.9':
bpo-41939: Fix test_site.test_license_exists_at_url() (GH-22559) (#22567)
https://github.com/python/cpython/commit/10b4136bfa5878c058753e1d1bd091e0f7e1ba40

New changeset 78bddc7794a629614dc84e195b9117416ae97ed3 by Miss Skeleton (bot) in branch '3.8':
bpo-41939: Fix test_site.test_license_exists_at_url() (GH-22559)
https://github.com/python/cpython/commit/78bddc7794a629614dc84e195b9117416ae97ed3

New changeset 6a48518e8dac3521ff387ee67cdf33783114a257 by Ned Deily in branch 'master':
bpo-41939: always enable test_site.test_license_exists_at_url (GH-22688)
https://github.com/python/cpython/commit/6a48518e8dac3521ff387ee67cdf33783114a257

New changeset 8b4642d3288e187faad24283c949ecf53fecad5b by Miss Skeleton (bot) in branch '3.8':
bpo-41939: always enable test_site.test_license_exists_at_url (GH-22688)
https://github.com/python/cpython/commit/8b4642d3288e187faad24283c949ecf53fecad5b

New changeset 881a13cad534cf3fe4474579c22235a15d3ba27b by Miss Skeleton (bot) in branch '3.9':
bpo-41939: always enable test_site.test_license_exists_at_url (GH-22688)
https://github.com/python/cpython/commit/881a13cad534cf3fe4474579c22235a15d3ba27b

Thanks for the fix, Victor.

Sometime after filing this, I remembered that we had seen a similar problem some years ago and found bpo-21572 in which I had changed Lib/site.py to no longer use a release-dependent URL for the license file - which is why the check to skip if on a pre-release had been added previously to test_license_exists_at_url. So I should have removed the skip in test_site at the same time as it is no longer needed and hides problems like this until the final release.  PR 22688 and its backports remove the skip.

You're welcome ;-)