Set stacklevel when warning about depreciated function views

[paulb@…]

It would be handy if the depreciation warning messages gave a clue as to the cause of warning. This can be done by adding severity to the warning.

Eg

#django.views.generic.simple.py

import warnings
warnings.warn(
    'Function-based generic views have been deprecated; use class-based views instead.',
    DeprecationWarning, stacklevel=2
)

[aaugustin]

Actually, it may be a good idea to always set stacklevel appropriately (is 2 always the "right" value?) wherever Django raises a DeprecationWarning.

[nickmartini]

All DeprecationWarning are now stacklevel=2. I made no changes to PendingDeprecationWarning.

​https://github.com/django/django/pull/349

[aaugustin]

This is actually a duplicate of #18127, the same pull request was posted to both tickets.