I've just updated the main function to replace the deprecated btrfs.utils.mounted_filesystems method with btrfs.utils.mounted_filesystem_paths as found in the 2019 release of python-btrfs.

The old main function was:

def main():
    for fs in btrfs.utils.mounted_filesystems():
        devices = fs.devices()

        if len(sys.argv) > 1 and sys.argv[1] == 'config':
            munin_config(fs, devices)
        else:
            munin_values(fs, devices)

I have directly updated the snippet as this doesn't change any other thing in the algorithms.