gh-156031: Restore urllib.request._opener in test_httpservers

[ayaangazali]

test_httpservers leaves urllib.request._opener installed for the rest of the test run, so regrtest flags it as altering the execution environment. Details and the trace are in gh-156031.

Short version: CommandLineRunTimeTestCase.fetch_file() calls urlopen() with no SSL context for the plain HTTP cases, and urlopen() only builds a throwaway opener when a context is passed. Otherwise it assigns the module global:

elif _opener is None:
    _opener = opener = build_opener()

test_urllib already guards the same global with self.addCleanup(urllib.request.urlcleanup) in four places, and urlcleanup() resets _opener, so this uses the same pattern rather than inventing a new one.

Verifying

Repro takes about two seconds. test_urllib runs first only to get urllib.request into sys.modules, which is what makes regrtest watch the resource at all:

./python.exe -m test test_urllib test_httpservers

Before:

Warning -- urllib.requests._opener was modified by test_httpservers
test_httpservers failed (env changed)

After: SUCCESS, same 209 tests run.

Also ran the whole suite sequentially, which is the only mode that detects this, before and after:

	before	after
tests OK	459	460
env changed	1 (test_httpservers)	0
failed	1 (test_struct)	1 (test_struct)

test_httpservers goes from failed (env changed) to passed. The test_struct failure is unrelated and present before my change too, so I left it alone. pre-commit passes on the touched file.

Worth noting why this has not turned up in CI: save_env.py only watches the resource when urllib.request is already imported, because try_get_module() raises SkipTestEnvironment otherwise. Under -j each test file gets a fresh subprocess, so the module is not in sys.modules when regrtest snapshots the environment and the resource is never tracked. Running test_httpservers on its own is clean for the same reason. It only shows up sequentially, after something earlier has imported urllib.request.

No test added. The fix is test-only hygiene and the suite run above is the check, so a new test would just restate it.

apologies if any of this is off. I found it by running the suite sequentially, traced the cause myself, and talked the fix choice over with Claude Code before settling on matching the existing test_urllib pattern. happy to be corrected on any of it. freshman in college, doing what I can to help out :)

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