<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def are_requirements_changed(config):
 """Check if any of the requirement files used by testenv is updated. :param tox.config.TestenvConfig config: Configuration object to observe. :rtype: bool """

    deps = (dep.name for dep in config.deps)

    def build_fpath_for_previous_version(fname):
        tox_dir = config.config.toxworkdir.strpath
        envdirkey = _str_to_sha1hex(str(config.envdir))
        fname = '{0}.{1}.previous'.format(fname.replace('/', '-'), envdirkey)
        return os.path.join(tox_dir, fname)

    requirement_files = map(parse_requirements_fname, deps)
    return any([
        is_changed(reqfile, build_fpath_for_previous_version(reqfile))
        for reqfile in requirement_files
        if reqfile and os.path.isfile(reqfile)])