[red-knot] Fix str(…) calls (#17163)

## Summary

The existing signature for `str` calls had various problems, one of
which I noticed while looking at some ecosystem projects (`scrapy`,
added as a project to mypy_primer in this PR).

## Test Plan

- New tests for `str(…)` calls.
- Observed reduction of false positives in ecosystem checks

.github/workflows/mypy_primer.yaml

@@ -68,7 +68,7 @@ jobs:
             --type-checker knot \
             --old base_commit \
             --new "$GITHUB_SHA" \
-            --project-selector '/(mypy_primer|black|pyp|git-revise|zipp|arrow|isort|itsdangerous|rich|packaging|pybind11|pyinstrument|typeshed-stats)$' \
+            --project-selector '/(mypy_primer|black|pyp|git-revise|zipp|arrow|isort|itsdangerous|rich|packaging|pybind11|pyinstrument|typeshed-stats|scrapy)$' \
             --output concise \
             --debug > mypy_primer.diff || [ $? -eq 1 ]