Replace Travis-CI with Github Actions

Travis-CI stopped working, switching over to Github Actions for automated
unit tests.

This removes the used of 'coveralls' in the CI pipeline (may come back
at some point), and adds testing on pypy 3.8 and 3.9.

.github/workflows/tests.yml

@@ -0,0 +1,32 @@
+name: Test Python-RSA
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version:
+          - "3.7"
+          - "3.8"
+          - "pypy3.8"
+          - "3.9"
+          - "pypy3.9"
+          - "3.10"
+          - "3.11"
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install poetry
+          poetry install
+      - name: Run tox
+        # Run tox using the version of Python in `PATH`
+        run: poetry run tox -e py