Added GHA tests

.github/workflows/tests.yaml

+---
+name: tests
+on:
+  push:
+  pull_request:
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python:
+          - '3.11'
+          - '3.10'
+          - '3.9'
+          - '3.8'
+          - '3.7'
+          - 'pypy-3.7'
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python }}
+      - uses: actions/cache@v1
+        with:
+          path: ~/.cache/pip
+          key: pip|${{ hashFiles('setup.py') }}|${{ hashFiles('tox.ini') }}
+      - run: pip install tox
+      - run: tox -e py