test_urllib2 alters the execution environment (unsetting `no_proxy`)

Bug report

Bug description:

Running the Python-3.13.1 test suite with make test, and:

1 test altered the execution environment (env changed):
    test_urllib2

The offending code pattern in several test cases is:

os.environ['no_proxy'] = '*'
# ... ...
del os.environ['no_proxy']

Thus if no_proxy already exists in the environment variable, these tests will remove it.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-132584
• gh-132585

首先使用 make test 运行 CPython 测试套件，并检查 test_urllib2，重点关注设置和删除 no_proxy 的用例。当测试保持现有的 no_proxy 环境值不变，且测试套件报告执行环境未发生变化时，该修改就完成了；关联的 PR gh-132584 和 gh-132585 表明相关工作已经在进行中。