internal: Code reorganization to put less stuff in `sources.base`. Interfaces that sources can implement are in `sources.interfaces`. The default page source is in `sources.default`. The `SimplePageSource` is gone since most subclasses only wanted to do *some* stuff the same, but *lots* of stuff slightly different. I may have to revisit the code to extract exactly the code that's in common.

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build node_modules venv tmp* {args}