Update Flake8 repo (#683)

Flake8 has moved away from gitlab to github.
Updating the repo path in the pre commit config.

.pre-commit-config.yaml

@@ -23,7 +23,7 @@ repos:
     hooks:
       - id: mypy
         name: Check type hint using mypy
-  - repo: https://gitlab.com/pycqa/flake8
+  - repo: https://github.com/pycqa/flake8
     rev: 3.8.4
     hooks:
       - id: flake8