opencensus-ext-django does not support django 4.1

[giltal1]

Describe your environment.
Django = "4.1"
opencensus-ext-django = "0.7.6"

Steps to reproduce.
Run a normal Django application

What is the expected behavior?
Django works normally and opencensus middleware works normally

What is the actual behavior?
Middleware fails with the following error, causing the Django app to not work
'OpencensusMiddleware' object has no attribute '_is_coroutine'

Additional context.
The init method does not call super() and therefore the middleware fails. please add a call to super()

[JeremyVriens]

Hi @giltal1 since we are experiencing the same issue and you didn't mention picking it up, I have created the following PR: #1159