test_urllib2.HandlerTests.test_redirect_no_path fails when proxy environment variables are set

Bug report

Bug description:

Running:

./python -m unittest test.test_urllib2.HandlerTests.test_redirect_no_path

can fail in environments where proxy-related variables are set (http_proxy, https_proxy, etc.).

The test output shows below:

F
======================================================================
FAIL: test_redirect_no_path (test.test_urllib2.HandlerTests.test_redirect_no_path)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yansendao/git/cpython/Lib/test/test_urllib2.py", line 1400, in test_redirect_no_path
    fp = urllib.request.urlopen("http://python.org/path")
  File "/home/yansendao/git/cpython/Lib/urllib/request.py", line 187, in urlopen
    return opener.open(url, data, timeout)
           ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/home/yansendao/git/cpython/Lib/urllib/request.py", line 489, in open
    response = self._open(req, data)
  File "/home/yansendao/git/cpython/Lib/urllib/request.py", line 506, in _open
    result = self._call_chain(self.handle_open, protocol, protocol +
                              '_open', req)
  File "/home/yansendao/git/cpython/Lib/urllib/request.py", line 466, in _call_chain
    result = func(*args)
  File "/home/yansendao/git/cpython/Lib/urllib/request.py", line 1352, in http_open
    return self.do_open(http.client.HTTPConnection, req)
           ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/yansendao/git/cpython/Lib/urllib/request.py", line 1323, in do_open
    h.request(req.get_method(), req.selector, req.data, headers,
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
              encode_chunked=req.has_header('Transfer-encoding'))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/yansendao/git/cpython/Lib/test/test_urllib2.py", line 1395, in request
    self.assertEqual(url, next(urls))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
AssertionError: 'http://python.org/path' != '/path'
- http://python.org/path
+ /path


----------------------------------------------------------------------
Ran 1 test in 0.010s

FAILED (failures=1)

Expected behavior
This test should validate relative redirect behavior (Location: ?query) independently of external shell/network configuration. It should pass regardless of whether proxy env vars are present.

Root cause
urllib.request.urlopen() honors proxy settings from environment variables.
When proxying is enabled, the HTTP request target format changes to absolute-form, which does not match this test’s path-only assertion.

Proposed fix
Make the test environment-independent by clearing proxy-related environment variables inside test_redirect_no_path using EnvironmentVarGuard (both lower/upper case variants, plus no_proxy/NO_PROXY).

Why proxy configuration matters when running tests
Some environments (corporate/campus networks, CI runners) require proxy settings for outbound connectivity; without them, networked tests may fail due to connection/timeouts.
However, logic-focused unit tests should not depend on external proxy state, so they should explicitly isolate themselves from proxy env effects.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-148920

從 Lib/test/test_urllib2.py 中的 HandlerTests.test_redirect_no_path 開始，檢查測試如何使用 EnvironmentVarGuard。在設定和未設定 proxy 變數的情況下執行 ./python -m unittest test.test_urllib2.HandlerTests.test_redirect_no_path。完成的標準是測試驗證相對重新導向，並且不受 proxy 環境設定影響而通過。