Fix Github actions (#727)

* Bump checkout to v3 and setup-python to v4

* Remove unused branches

* Fix ubuntu at v20.04

.github/workflows/flake8.yaml

@@ -2,20 +2,20 @@ name: Python flake8
 
 on:
   push:
-    branches: [ main, master, dev, development ]
+    branches: [ master, dev ]
   pull_request:
-    branches: [ main, master, dev, development ]
+    branches: [ master, dev ]
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies