Deprecate self in module, package and session scoped fixtures

[nuemaan]

Closes #14765.

#14071 deprecated instance-method fixtures at class scope, since attributes set on self never reach the tests. Each test runs on a fresh instance while the fixture runs once per class. module, package and session scope have exactly the same problem and were still silent, so this extends the deprecation to cover them.

Confirmed the breakage is real before writing the warning. With scope="module" or scope="session", a fixture that sets self.attr leaves the test seeing nothing:

E       AssertionError: assert None == 'set-by-fixture'

The message differs from the class-scoped one. @classmethod is what the class-scoped warning recommends, but for these wider scopes the class body is not a useful place to hang state either, so the new message points at @staticmethod. Worth noting that @classmethod does happen to work at these scopes, so if you would rather the message mention both, that is a one-line change.

Function scope is deliberately untouched. The fixture and the test share an instance there, so self behaves as people expect.

Four fixtures in our own test suite matched the pattern:

• testing/test_legacypath.py TestFixtureRequestSessionScoped.session_request
• testing/test_conftest.py TestConftestValueAccessGlobal.basedir
• testing/test_pathlib.py TestImportPath.path1
• testing/python/fixtures.py TestRequestSessionScoped.session_request

Every one of them took self without using it, which is some evidence the deprecation catches accidental instance methods rather than deliberate ones. They are now staticmethods. TestImportPath.path1 called self.setuptestfs, which does not use self either, so that helper became a staticmethod as well.

Verification

• testing/deprecated_test.py, 23 passed, including 7 new
• Full testing/ suite with -n 8, 3761 passed against 3754 on unmodified main. Same 1 failure and same 5 errors appear on both, all from optional deps missing in my environment, attrs among them
• ruff check, ruff format --check, and mypy on the changed files, all clean
• Reverted the two src/ files and re-ran the new tests to confirm they fail without the change, rather than passing regardless

[RonnyPfannschmidt]

#14774