Streamlining requirements

[mataton]

Right now, skbio has four separate files for our requirements. The reason for this is to minimize installations for various parts of the CI workflow. For instance, the documentation portion of the CI workflow will only install packages in ci/requirements.doc.txt. However having the requirements separated like this makes the commands for setting up a development environment bulky.

conda create -n skbio-dev -c conda-forge --file ci/conda_requirements.txt --file ci/requirements.test.txt --file ci/requirements.lint.txt --file ci/requirements.doc.txt

Consolidating the requirements into a single file may be more optimal, or perhaps there is a more elegant solution.