Add GitHub action to run unit tests

Based on:
https://github.com/cookiecutter/cookiecutter/blob/master/.github/workflows/coverage.yml

.github/workflows/test.yml

@@ -0,0 +1,25 @@
+name: Build
+
+on: [push]
+
+jobs:
+  run_unit_tests:
+    runs-on: ubuntu-latest
+    steps:
+
+    - name: Git repository checkout
+      uses: actions/checkout@v2
+
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v1
+      with:
+        python-version: "3.9"
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install tox virtualenv
+
+    - name: Run unit tests
+      run: |
+        make test

README.md

@@ -27,3 +27,5 @@ FearGreedIndex(
 ```
 
 Requests to CNN's website are locally [cached](https://pypi.org/project/requests-cache/) for 1m.
+
+![Test workflow](https://github.com/vterron/fear-and-greed/actions/workflows/test.yml/badge.svg)