github actions: Merge workflows for linting to lint.yml

.github/workflows/docslint.yml

@@ -1,16 +0,0 @@
-name: Lint the format of document
-
-on: [push, pull_request]
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v2
-    - name: Set up Python
-      uses: actions/setup-python@v1
-    - name: Install dependencies
-      run:  pip install -U tox
-    - name: Run Tox
-      run:  tox -e docslint

.github/workflows/lint.yml

@@ -1,10 +1,14 @@
-name: Lint source code (flake8)
+name: Lint source code
 
 on: [push, pull_request]
 
 jobs:
   build:
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        tool: [docslint, flake8, mypy]
 
     steps:
     - uses: actions/checkout@v2
@@ -15,4 +19,4 @@ jobs:
     - name: Install dependencies
       run:  pip install -U tox
     - name: Run Tox
-      run:  tox -e flake8
+      run:  tox -e ${{ matrix.tool }}

.github/workflows/mypy.yml

@@ -1,16 +0,0 @@
-name: Lint source code (mypy)
-
-on: [push, pull_request]
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v2
-    - name: Set up Python
-      uses: actions/setup-python@v1
-    - name: Install dependencies
-      run:  pip install -U tox
-    - name: Run Tox
-      run:  tox -e mypy