loader.pypi.tests: Refactor using loader-core mixins
Clean up duplication in tests code by reusing the loader-core's dedicated mixins.
Also:
- Cleanup swh.loader.pypi.model remnant module
- Fix typos
- Fix a missing dangling temporary folder
Related swh-loader-core#1238 (closed)
Depends on swh-loader-core!4 (closed)
Test Plan
make test
Migrated from D475 (view on Phabricator)