lint: isort pre-commit & misc improvements (#3749)

* test: clean flake8 config root clutter

* test: pre-commit: add isort

Compatible with black (see https://github.com/timothycrosley/isort/issues/694)

* lint: reorder imports

* lint: more complex imports

* lint: fix deepsource

* config: isort: add first party dvc

* tests: restyled: add isort

* temp commit to revert before merge

* Revert "temp commit to revert before merge"

This reverts commit 11db0fd8e95fe891597cbb9bf804480909b19e24.

tests/func/test_get.py

@@ -4,13 +4,13 @@ import os
 import pytest
 
 from dvc.cache import Cache
-from dvc.main import main
+from dvc.compat import fspath
 from dvc.exceptions import PathMissingError
-from dvc.repo.get import GetDVCFileError
+from dvc.main import main
 from dvc.repo import Repo
+from dvc.repo.get import GetDVCFileError
 from dvc.system import System
 from dvc.utils.fs import makedirs
-from dvc.compat import fspath
 from tests.utils import trees_equal