Set up GitHub Action for linting

This will give faster linting feedback since we can skip compilation.

.github/workflows/lint.yml

@@ -0,0 +1,27 @@
+name: lint
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Setup Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9.0-beta.4 - 3.9.0
+
+    - name: Install tox
+      run: python -m pip install tox
+
+    - name: Main check
+      run: python -m tox -e lint

tox.ini

@@ -7,14 +7,16 @@ isolated_build = True
 deps =
     Cython
     scipy
-    flake8
     pytest-cov
-commands =
-    flake8
-    pytest
+commands = pytest
 setenv = CYTHON_TRACE = 1
 passenv = TRAVIS
 
+[testenv:lint]
+skip_install = true
+deps = flake8
+commands = flake8
+
 [flake8]
 filename = *.pxd, *.pyx, *.py
 hang-closing = True