We had them split according the separate use-cases:
- testing
- doc-gen
- wire-gen
But that was causing new contributors to miss some dependencies when they
first got hacking. So this consolidates all of our own dependencies in a root
requirements.txt, with the notable exception of `pyln-client`, `pyln-testing`
and `pyln-proto` which are distributed as PyPI modules and therefore have
their own dependencies that need to be tracked in the module root.
Closes#3518
b68066e8e8