pre-commit: replace isort mirror with isort upstream (#2118)

The isort pre-commit mirror has been deprecated. This change updates
configuration to use the upstream package repository instead of the
mirror.

.pre-commit-config.yaml

@@ -9,8 +9,8 @@ repos:
     hooks:
       - id: flake8
 
-  - repo: https://github.com/pre-commit/mirrors-isort
-    rev: v4.3.21
+  - repo: https://github.com/timothycrosley/isort
+    rev: 4.3.21
     hooks:
       - id: isort
         additional_dependencies: [toml]