swev-id: django__django-16938 - Clear select_related before serializer m2m iteration

[casey-brooks]

Summary

• clear queryset select_related/prefetch_related before only('pk') when serializing M2M relations
• add regression coverage exercising JSON and XML serializers with related managers using select_related
• ensure serialization emits related PKs without FieldError (Fixes Serialization of m2m fails with select_related default manager (FieldError) — swev-id: django__django-16938 #532)

Testing

• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py serializers.test_json serializers.test_xml --parallel=1
• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages flake8 django/core/serializers/python.py django/core/serializers/xml_serializer.py tests/serializers/test_json.py tests/serializers/test_xml.py

[casey-brooks]

Tests & Lint Summary

• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py serializers.test_json serializers.test_xml --parallel=1 (66 passed, 0 failed, 0 skipped)
• PYTHONPATH=$PWD:$HOME/.nix-profile/lib/python3.11/site-packages flake8 django/core/serializers/python.py django/core/serializers/xml_serializer.py tests/serializers/test_json.py tests/serializers/test_xml.py (lint passed with no issues)

[noa-lucent]

Looks good to me.

[casey-brooks]

Local validation

• ⚠️ .venv/bin/tox -e py3 -- serializers.test_json serializers.test_xml --parallel=1 (failed: pylibmc build requires libmemcached/cyrus-sasl/zlib headers even after installing toolchain; fell back to direct runtests.py per instructions)
• ✅ PYTHONPATH=$PWD DJANGO_SETTINGS_MODULE=tests.test_sqlite /workspace/venv/bin/python tests/runtests.py serializers.test_json serializers.test_xml --parallel=1
• ⚠️ tox -e flake8 (failed: repo contains intentional syntax-error fixture tests/test_runner_apps/tagged/tests_syntax_error.py; flake8 3.9.x crashes under Python ≥3.10 despite NOQA, so ran scoped lint instead)
• ✅ /workspace/py310venv/bin/flake8 django/core/serializers/python.py django/core/serializers/xml_serializer.py tests/serializers/test_json.py tests/serializers/test_xml.py
• ✅ /workspace/py310venv/bin/isort --check-only --diff django/core/serializers/python.py django/core/serializers/xml_serializer.py tests/serializers/test_json.py tests/serializers/test_xml.py

[noa-lucent]

Clearing select_related()/prefetch_related() before the PK-only iterator prevents the deferred-field FieldError without regressing the optimization, and the new JSON/XML regression tests cover managers that default select_related and assert the expected related PK output. Looks good.