Remove superseded ExtendedQuerySet as it's functionality is built into Django since 1.7

[lociii]

Fixes #562

ExtendedQuerySet provided a custom implementation of update_or_create before the release of Django 1.7. Since then Django added it to the base QuerySet.
Only difference is that update_or_create of Django's QuerySet returns the object and creation state whereas the custom implementation of django-celery-beat only returned the object. Therefore ModelEntry.from_entry had to be adjusted.

As a cleanup, I removed all managers and introduced a single queryset for PeriodicTask.

[auvipy]

builds are failing. also some additional tests would be great

[lociii]

builds are failing. also some additional tests would be great

Builds are fixed now as far as I can influence them.
What kind of tests are you looking for? I can test PeriodicTaskQuerySet but that seems pretty pointless to me.

[auvipy]

it's better to verify with automated test

[auvipy]

can you add a little test for the queryset?

[954-Ivory]

can you add a little test for the queryset?

This PR just use django native model function, to replace custom function.
It should not to add additional test code.

I think this PR is better in django version>1.7:
Because it will call native function, the native function will call with transaction.atomic, it's safer.

But I don't know does django has update_or_create in version <1.7.
If there has not update_or_create, some user use django-celery-beat with django<1.7 will got the error.

[auvipy]

this will be django 2.2+ only