add context based handlers for ptb12

[lukruh]

since the original repository seems to be abbandoned, we use the own fork now

[pep8speaks]

Hello @lukruh! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file django_telegrambot/apps.py:

Line 208:80: E501 line too long (93 > 79 characters)