tox: Start configuration to test on Py3.6 and Py3.7.

This caught several of the recent issues.

.gitignore

@@ -4,4 +4,5 @@ build/
 *.egg-info/
 .eggs
 .mypy_cache/
+.tox/
 __pycache__/

README.rst

@@ -10,8 +10,17 @@ Installation
 
 Of course, if you're familiar with Python development tools, you're welcome to install the module in a virtualenv, somewhere else, etc.
 
-Running tests
+Running all tests
--------------
+-----------------
+
+The project comes with a Tox configuration that defines all the tests we expect to pass, across all the different environments we support. To run them all, run::
+
+  tox
+
+The next sections describe how to run them standalone in your own development environment, for faster iteration or simpler isolation of problems.
+
+Running unit tests
+------------------
 
 Run::
 

tox.ini

@@ -0,0 +1,11 @@
+[tox]
+envlist = py36,py37
+
+[testenv]
+# Beancount includes type declarations but not the `py.typed` flag file mypy
+# is looking for to know that. Create it ourselves.
+commands_pre = python -c 'import beancount, pathlib; pathlib.Path(beancount.__file__).with_name("py.typed").touch()'
+
+commands =
+  ./setup.py test
+  ./setup.py typecheck