Replace Poetry GH action with plain Python

.github/workflows/lint.yml

@@ -1,20 +1,28 @@
 name: Lint
+
 on:
-  push:
   pull_request:
-    branches: [ master ]
 
 jobs:
   build:
+
     runs-on: ubuntu-latest
+
     steps:
-    - name: Install
-      uses: abatilo/[email protected]
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.8
+      uses: actions/setup-python@v1
       with:
-        python_version: 3.8.0
-        poetry_version: 1.0.5
-        args: install
-    - name: Lint
-      run:
-        poetry run mypy c4p2n
+        python-version: 3.8
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install poetry>=1.0.0
+        poetry config virtualenvs.create false
+        poetry install
+    - name: Lint with black
+      run: |
         poetry run black --check c4p2n
+    - name: Check types with mypy
+      run: |
+        poetry run mypy c4p2n