Add setuptools to requirements for Python 3.12+ compatibility

Python 3.12 no longer bundles setuptools in virtual environments.
Pyramid imports pkg_resources from setuptools, causing CI to fail with
ModuleNotFoundError: No module named 'pkg_resources'.

CLAUDE.md

@@ -127,6 +127,32 @@ alembic -c data/development.ini history
 
 ### Important Migration Notes
 
+**Idempotent Migrations**: `make init-db` creates all tables from models, so migrations that run afterward must not fail if tables/columns already exist. Always guard `create_table` with `_table_exists` and `add_column` with `_column_exists`:
+
+```python
+def _table_exists(name):
+    conn = op.get_bind()
+    result = conn.execute(
+        sa.text("SELECT name FROM sqlite_master WHERE type='table' AND name=:name"),
+        {"name": name},
+    )
+    return result.fetchone() is not None
+
+
+def _column_exists(table, column):
+    conn = op.get_bind()
+    result = conn.execute(sa.text(f"PRAGMA table_info({table})"))
+    return any(row[1] == column for row in result.fetchall())
+
+
+def upgrade():
+    if not _table_exists("mps_new_table"):
+        op.create_table(...)
+
+    if not _column_exists("mps_shop", "new_column"):
+        op.add_column(...)
+```
+
 **SQLite Column Defaults**: When adding NOT NULL columns with defaults to existing tables in SQLite, use `server_default` with raw SQL values:
 
 ```python

requirements.py3.txt

@@ -1,6 +1,10 @@
 # python 2 -> 3 compat.
 six
 
+# Python 3.12+ no longer bundles setuptools in venvs.
+# Pyramid imports pkg_resources which lives in setuptools.
+setuptools
+
 # Pyramid with SQLAlchemy.
 plaster_pastedeploy
 pyramid