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_update.py

@@ -1,8 +1,9 @@
-import pytest
 import os
 
-from dvc.dvcfile import Dvcfile
+import pytest
+
 from dvc.compat import fspath, fspath_py35
+from dvc.dvcfile import Dvcfile
 
 
 @pytest.mark.parametrize("cached", [True, False])