[ci] Deprecate flake8 (#8409)

* [ci] Deprecate flake8

* Addressing @villebro's comments

.pre-commit-config.yaml

@@ -16,11 +16,21 @@
 #
 repos:
 -   repo: https://github.com/ambv/black
-    rev: stable
+    rev: 19.3b0
     hooks:
     -   id: black
         language_version: python3
 
+-   repo: https://github.com/asottile/seed-isort-config
+    rev: v1.9.3
+    hooks:
+    -   id: seed-isort-config
+
+-   repo: https://github.com/pre-commit/mirrors-isort
+    rev: v4.3.21
+    hooks:
+    -   id: isort
+
 -   repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v2.2.3
     hooks:
@@ -30,8 +40,3 @@ repos:
     -   id: check-added-large-files
     -   id: check-yaml
     -   id: debug-statements
-
--   repo: https://gitlab.com/pycqa/flake8
-    rev: 3.7.1
-    hooks:
-    -   id: flake8