Creating RequestSpec from flavor tries to use nonexistent attribute

[Dominik Danelski]

Description
===========
Calling RequestSpec.from_primitives, which in turn calls spec._from_flavor ends with "AttributeError: module 'nova.objects' has no attribute 'Flavor'" after the check "isinstance(flavor, objects.Flavor)"

Steps to reproduce
==================
* I directly imported RequestSpec via "from nova.objects.request_spec import RequestSpec" for testing purposes
* I called RequestSpec.from_primitives
* RequestSpec.from_primitives called RequestSpec._from_flavor to fill the missing information
* RequestSpec._from_flavor performed a check "isinstance(flavor, objects.Flavor)", which resulted in the exception

Expected result
===============
I should have received a RequestSpec object or another error if the data provided was incorrect.

Actual result
=============
The exception "AttributeError: module 'nova.objects' has no attribute 'Flavor'" is raised.

Traceback:
RequestSpec.from_primitives(scheduler_connector_nova_rpc._context, dictionary_from_json, {})
  File "/home/eouser/placement-simulation-tool/venv/lib/python3.10/site-packages/nova/objects/request_spec.py", line 331, in from_primitives
    spec._from_flavor(flavor)
  File "/home/eouser/placement-simulation-tool/venv/lib/python3.10/site-packages/nova/objects/request_spec.py", line 249, in _from_flavor
    if isinstance(flavor, objects.Flavor):
AttributeError: module 'nova.objects' has no attribute 'Flavor'

Environment
===========
I use Nova 21.2.4 from PyPi, Python 3.10 on Ubuntu 20.04.

[Uggla (rene-ribaud)]

Thanks for reporting this bug. It is legit, but it needs to be discussed as the RequestSpec.from_primitives method is deprecated and should be removed at some point. So we need to discuss whether it is worth fixing or simply removing the method.