Moving towards removing "requirements" files.

[molpopgen]

Currently, we specify dependencies in more than one place:

1. pyproject.toml
2. file in in requirements/

This redundancy leads to the obvious problems.
We can solve the problem by:

1. Using optional features in pyproject.toml:

[project.optional-dependencies]
moments-popgen = ["moments-popgen>=1.2.0"]

These optional features can be for various tasks (development, CI, etc.)

2. We than then do editable local installs for CI that tell cmake to do the right thing:

python -m pip install -Ccmake.define.BUILD_PYTHON_UNIT_TESTS=On -Ccmake.define.BUILD_CPP_UNIT_TESTS=On -e .

(Currently, the above doesn't work.
The C++ test suite ends up in the cmake build directory.
But we could output it somewhere sensible instead?)

[molpopgen]

Closed via #1353