import email.mime.text

msg = email.mime.text.MIMEText('body text', _charset='utf-8')
msg['Subject'] = 'Re: generate_series(date,date)'
print(msg.as_string())

msg = email.mime.text.MIMEText('body text', _charset='utf-8')
msg['Subject'] = ('Re: '
                  'aaaaaaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbbbb '
                  'cccccccccccccccccccc dddddddddddddddddddd '
                  'generate_series(date,date) '
                  'eeeeeeeeeeeeeeeeeeee')
print(msg.as_string())