switch to gh actions

.github/workflows/lint.yml

@@ -0,0 +1,12 @@
+
+name: Lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - uses: psf/black@stable

.travis.yml

@@ -1,9 +0,0 @@
-language: python
-python:
-  - "3.6"
-  - "3.7"
-  - "3.8"
-install:
-  - pip install black
-script:
-  - black --check .