test_urllib2 alters the execution environment (unsetting no_proxy)

[xry111]

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

[graingert]

Ah could we use mock.patch.dict instead?