Remove unnecessary config for isort 5

[hugovk]

Follow up to #8902.

We can remove known_first_party and default_section = THIRDPARTY with isort 5.

https://pycqa.github.io/isort/docs/upgrade_guides/5.0.0/#module-placement-changes-known_third_party-known_first_party-default_section-etc says:

isort has completely rewritten its logic for placing modules in 5.0.0 to ensure the same behavior across environments. You can see the details of this change here. The TL;DR of which is that isort has now changed from default_section=FIRSTPARTY to default_section=THIRDPARTY. If you all already setting the default section to third party, your config is probably in good shape. If not, you can either use the old finding approach with --magic-placement in the CLI or old_finders=True in your config, or preferably, you are able to remove all placement options and isort will determine it correctly. If it doesn't, you should be able to just specify your projects modules with known_first_party and be done with it.

So default_section = THIRDPARTY is now the default and can be removed from config.

---

You can see the details of this change here.

says:

• FIRSTPARTY: import name starts with .
• FIRSTPARTY: import code exists within the files isort is told to sort.
• FIRSTPARTY: import code exists in same directory as any files isort is told to sort.

So pip and tests match first party and this can now be removed from config:

known_first_party =	
    pip	
    tests

[pradyunsg]

Aye! This is nice!