Parse ``requirements.txt`` at ``path``.

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