tox: add & document tox usage

This makes it easy to run all the tests against multiple versions
of Python.  We want to make sure Python 2.7 & 3.6+ work.

Change-Id: Ia7b16eb46a2aa7c240f03bb291987fa8cb215267
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247174
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

tox.ini

@@ -0,0 +1,22 @@
+# Copyright 2019 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# https://tox.readthedocs.io/
+
+[tox]
+envlist = py27, py36, py37, py38
+
+[testenv]
+deps = pytest
+commands = {toxinidir}/run_tests