Remove unmaintained pytest-snail dependency

Remove pytest-snail from dev requirements. This plugin is unmaintained
and causes deprecation warnings with modern pytest. The @pytest.mark.slow
marker it provides is not used anywhere in the codebase.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: rsokl

requirements/dev.txt

@@ -12,7 +12,6 @@ packaging
 pre-commit
 pytest==8.3.5
 pytest-benchmark
-pytest-snail
 read-version
 setuptools
 towncrier