gh-148919: Make test_redirect_no_path independent of proxy environment variables

[sendaoYan]

Summary
test.test_urllib2.HandlerTests.test_redirect_no_path can fail when proxy-related environment variables are set (for example, http_proxy / HTTP_PROXY).

This change makes the test self-contained by clearing proxy-related environment variables within the test via test.support.os_helper.EnvironmentVarGuard.

Rationale
The test is intended to verify relative redirect behavior (Location: ?query) and should not depend on external shell/network proxy configuration.

When proxy env vars are active, request targets may be sent in absolute-form (http://python.org/path) instead of origin-form (/path), which breaks the current assertion and introduces environment-dependent failures.

Changes
Update Lib/test/test_urllib2.py in test_redirect_no_path:
Add EnvironmentVarGuard
Remove proxy-related variables (lowercase and uppercase variants, including no_proxy / NO_PROXY) for the duration of the test

• Issue: test_urllib2.HandlerTests.test_redirect_no_path fails when proxy environment variables are set #148919

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[sendaoYan]

Test-fix only, I think this PR should add a label 'skip news'.