move packaging and installation requirements to tests/requirements.txt

[jhkennedy]

@yunjunz this will fix #76, and shouldn't require any docs or CI/CD changes.

Basically, this:

• changes requirements.txt to be runtime dependencies only
• changes tests/requirements.txt to be all additional dependencies for development (building, packaging, testing)

which is fine overall (if you're developing, you should be testing!) and coherent with the docs.

Strictly speaking, I don't think you need to list setuptools, setuptools_scm, or wheel as development dependencies as pip will do isolated builds if a pyproject.toml is present and pull the build dependencies from the [build-system] requirements listed there.

But, I do like having setuptools_scm in the dev environment so I can easily check what version it thinks the project is on with:

python -m setuptools_scm

If you're going to keep them, however, it's probably worth including the same pins in the requirements.txt files as pyproject.toml:

requires = ["setuptools<60.0", "setuptools_scm[toml]>=6.2", "numpy<1.23.0", "wheel"]

[yunjunz]

This looks clean and nice. Thank you @jhkennedy.

Thanks for the explanation on setuptools and wheel. They are now gone.

Merge it if you think it's ready please.