v1.4

• Admin: display and filter verified songs
• Dependencies: update Django to 3.1
• Dependencies: update django-bootstrap3 to 12.0
• Dependencies: update Python to 3.8
• Dependencies: specify the minimal required Python version
• Templates: get translated difficulty in the song template
• Tests: add python 3.7 and 3.8 compatibility
• Translations: use lazy translations in models to get translated text in templates