setup.py: add iso639

requirements.txt

 lxml==4.6.5
-iso639==0.1.4

setup.py

@@ -34,6 +34,6 @@ setup(name='archive-hocr-tools',
       include_package_data=True,
       install_requires=['lxml'],
       extras_require={
-          'epub': ['ebooklib==0.17.1', 'internetarchive-deriver-module==1.0.1'],
+          'epub': ['ebooklib==0.17.1', 'internetarchive-deriver-module==1.0.1', 'iso639==0.1.4'],
       },
       package_data={'hocr': ['data/*']})