Test `pipreqs` to obtain requirements from code base

[ruestefa]

I recently stumbled upon pipreqs (see #33), a tool that scans a code base and creates a requirements.txt file. This could be useful especially for legacy projects that don't have a requirements file yet, or periodically for bigger codes to keep the requirements complete and clean. I haven't tested it, but if it works as advertised, it could be a useful addition to dev-requirements.yml.

Since the blueprint has switched to conda YAML files for both (pinned) environments and (unpinned) requirements (#32), the output of pipreqs would not directly useful to a project. However, the list of pinned requirements could easily be converted by hand to a format that can be diff'd against requirements.yml to identify missing/obsolete packages in the latter, or to derive minimum versions from the pinned versions.

Possible workflow

• Run pipreqs, which creates a requirements.txt
• Compare this requirements.txt with requirements.yml
  • Insert - on each line
  • Convert the pinned dependencies (e.g., ==1.4.2) to minimum requirements (e.g., >=1.4)
  • Use your diff tool of choice
• Add missing dependencies, remove obsolete ones, and add/adapt minimum versions as necessary
• Remove the requirements.txt

TODOs

• Test pipreqs
• If successful, add to dev-requirements.yml
• Briefly document usage