docs: def parse_requirements(path):
      """Parse ``requirements.txt`` at ``path``."""
      requirements = []
      with open(path, "rt") as reqs_f:
          for line in reqs_f:
              line = line.strip()
              if line.startswith("-r"):
                  fname = line.split()[1]
                  inner_path = os.path.join(os.path.dirname(path), fname)
                  requirements += parse_requirements(inner_path)
              elif line != "" and not line.startswith("#"):
                  requirements.append(line)
      return requirements