Adding flake8 to GHA and pre-commit. Fixing missing flake8

.github/workflows/style-checks.yml

@@ -1,6 +1,6 @@
 name: style checks
-# just formatting for now
-# TODO: add isort and flake8 later
+# just formatting and flake8 for now
+# TODO: add isort later
 
 on:
   pull_request:
@@ -20,8 +20,8 @@ jobs:
 
       - name: Install dependencies with pip
         run: |
-          pip install black
+          pip install black flake8
 
       # - run: isort --check-only .
       - run: black --check .
-      # - run: flake8
+      - run: flake8

.pre-commit-config.yaml

@@ -8,3 +8,10 @@ repos:
     language: system
     entry: black
     types: [python]
+
+  - id: flake8
+    name: flake8
+    stages: [commit]
+    language: system
+    entry: flake8
+    types: [python]