Use self instead of SimpleMemoryBackend

[and-semakin]

What do these changes do?

Use self instead of direct links to SimpleMemoryBackend class. This PR doesn't affect any functionality but allows users of the library inherit SimpleMemoryBackend and change its behaviour. Otherwise, changing the behavior is really difficult to do.

Are there changes in behavior for the user?

No.

Related issue number

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [x] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[Fogapod]

This is not a correct fix since you still use global cache and handlers. They are not set on init.

EDIT: Sorry, I misunderstood the purpose of this PR initially. I just made a similar one that actually moves state to instance. This is otherwise a positive change

[and-semakin]

@Fogapod exactly, I didn’t even try to fix the issue with global cache state. My only purpose here was just to make inheritance easier. It’s just one little step in the correct direction.

[Dreamsorcerer]

Looks good, sorry for the delay. Could you update with latest master and ensure the tests are all passing. Thanks.

[Dreamsorcerer]

I think this is solved now with #562. Please reopen if not.