ci: Switch from Travis to GitHub Actions

- Travis stopped providing unlimited free minutes to free software projects,
- Most zopefoundation/* projects already switch to GitHub actions for that reason,
- Despite tempstorage being dormant for a long time with no push/PR
  activity, Travis did not run any tests at all for https://github.com/zopefoundation/tempstorage/pull/16

.github/workflows/tests.yml

+# based on https://github.com/zopefoundation/meta/blob/master/config/default/tests.yml.j2
+name: tests
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '0 12 * * 0'  # run once a week on Sunday
+  # Allow to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+jobs:
+  build:
+    strategy:
+      # We want to see all failures:
+      fail-fast: false
+      matrix:
+        os:
+        - ubuntu
+        config:
+        # [Python version, tox env]
+        - ["2.7",   "py27"]
+        - ["3.5",   "py35"]
+        - ["3.6",   "py36"]
+        - ["3.7",   "py37"]
+        - ["3.8",   "py38"]
+        - ["3.9",   "py39"]
+        - ["pypy2", "pypy"]
+        - ["pypy3", "pypy3"]
+
+    runs-on: ${{ matrix.os }}-latest
+    name: ${{ matrix.config[1] }}
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.config[0] }}
+    - name: Pip cache
+      uses: actions/cache@v2
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ matrix.config[0] }}-${{ hashFiles('setup.*', 'tox.ini') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-${{ matrix.config[0] }}-
+          ${{ runner.os }}-pip-
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install tox
+    - name: Test
+      run: tox -e ${{ matrix.config[1] }}

.travis.yml

-language: python
-
-python:
-    - "2.7"
-    - "3.5"
-    - "3.6"
-    - "pypy"
-    - "pypy3.5"
-
-matrix:
-    include:
-        - python: "3.7"
-          dist: xenial
-
-install:
-    - pip install -U setuptools
-    - pip install zc.buildout
-    - buildout bootstrap
-    - buildout
-
-script:
-    - bin/test -v1
-
-notifications:
-    email: false
-
-cache:
-  pip: true
-  directories:
-    - eggs/

tox.ini

 [tox]
 envlist =
-    py27,py35,py36,py37,pypy,pypy3
+    py27,py35,py36,py37,py38,py39,pypy,pypy3
 
 [testenv]
 commands =