Slurper: add π-Base spaces and properties

[katjabercic]

Extend source coverage to π-Base.

Records to import

• Spaces: topological spaces (e.g. "Cantor set")
• Properties: topological properties (e.g. "compact", "Hausdorff")
• Theorems: named theorems

Note that spaces and properties live in distinct π-base namespaces. Use distinct identifier prefixes to avoid collisions?

Data source

π-Base publishes its full dataset as YAML in a public GitHub repo pi-base/data. Each record has a stable id, name, description, and URL on topology.pi-base.org.

Implementation sketch

1. Add a PI_BASE_TOPOLOGY choice to Item.Source.
2. Create web/slurper/source_pi_base_topology.py.
3. Add import_pi_base.py and clear_pi_base.py management commands.
4. Wire the importer into populate-db.
5. Add a basic test mirroring.