fix flake errors

src/common/email.py

 import os
-from django.core.mail import send_mail, EmailMessage
+from django.core.mail import EmailMessage
 import codecs
 import sys
 
 SENDER_EMAIL = os.getenv('SENDER_MAIL', 'kepzes@kszk.bme.hu')
-HOMEWORK_LINK = os.getenv('HOMEWORK_LINK', 'https://ujonc.kszk.bme.hu/homework')
+HOMEWORK_LINK = os.getenv(
+    'HOMEWORK_LINK', 'https://ujonc.kszk.bme.hu/homework')
 
 
 def get_full_name(user):
@@ -20,7 +21,8 @@ def read_email(name):
 def send_out_mail(subject, message, SENDER_EMAIL, receiver_email):
     if 'runserver' in sys.argv:
         # It is in development
-        print("Email sent from: " + str(SENDER_EMAIL) + " to: " + str(receiver_email))
+        print("Email sent from: " + str(SENDER_EMAIL) +
+              " to: " + str(receiver_email))
     else:
         email = EmailMessage(
             subject=subject,