Reconcile requirements files

[molpopgen]

We currently have too many requirements files, separated between the project root and doc/. Ideally, we'd put a minimal set of files in requirements/.

Part of the issue is that jupyter-book has very strict dependency pins, making it hard to reconcile all dependencies in a single file. For example, jupyter-book pins click to < 8 yet blacken-docs brings in black, which pins click to at least 8. (However, we are not using blacken-docs, so it can be removed. We do, however, use black internally for pretty-printing of objects. We may consider removing that and just documenting how to use asdict() methods for pretty-printing?)