Moved CI to GitHub workflows

The continuous integration ran on Travis. Unfortunately, Travis stopped
supporting open source projects on a free-for-all basis, so we had to
move the continous integration to GitHub.

.github/workflows/ci.yml

@@ -0,0 +1,53 @@
+name: CI
+
+on:
+  push:
+    branches: [ master, main ]
+    tags: [ "**" ]
+  pull_request:
+    branches: [ "**" ]
+
+jobs:
+  Execute:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.6, 3.7, 3.8]
+
+    steps:
+      - uses: actions/checkout@master
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: |
+          python3 -m pip install --upgrade pip
+          pip3 install --upgrade coveralls
+          pip3 install -e .[dev]
+
+      - name: Run checks
+        run: |
+          python3 precommit.py
+
+      - name: Upload Coverage
+        run: coveralls --service=github
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          COVERALLS_FLAG_NAME: ${{ matrix.python-version }}
+          COVERALLS_PARALLEL: true
+
+  Finish-Coveralls:
+    name: Finish Coveralls
+    needs: Execute
+    runs-on: ubuntu-latest
+    container: python:3-slim
+    steps:
+      - name: Finish Coveralls
+        run: |
+          pip3 install --upgrade coveralls
+          coveralls --finish --service=github
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

README.rst

@@ -1,7 +1,8 @@
 icontract
 =========
-.. image:: https://travis-ci.com/Parquery/icontract.svg?branch=master
-    :target: https://travis-ci.com/Parquery/icontract
+.. image:: https://github.com/Parquery/icontract/workflows/CI/badge.svg
+    :target: https://github.com/Parquery/icontract/actions?query=workflow%3ACI
+    :alt: Continuous integration
 
 .. image:: https://coveralls.io/repos/github/Parquery/icontract/badge.svg?branch=master
     :target: https://coveralls.io/github/Parquery/icontract