Add tox config for pyo3 itself

Makefile

@@ -17,6 +17,6 @@ default: test
 
 test:
 	cargo test $(CARGO_FLAGS)
-	pip install setuptools-rust pytest pytest-benchmark tox
+	pip install tox
-	cd examples/word-count && python setup.py install && pytest -v tests
+	cd examples/word-count && tox
 	cd examples/rustapi_module && tox

src/lib.rs

@@ -44,7 +44,8 @@
 //!
 //!     let locals = PyDict::new(py);
 //!     locals.set_item("os", py.import("os")?)?;
-//!     let user: String = py.eval("os.getenv('USER') or os.getenv('USERNAME')", None, Some(&locals))?.extract()?;
+//!     let code = "os.getenv('USER') or os.getenv('USERNAME') or 'Unknown'";
+//!     let user: String = py.eval(code, None, Some(&locals))?.extract()?;
 //!
 //!     println!("Hello {}, I'm Python {}", user, version);
 //!     Ok(())

tox.ini

@@ -0,0 +1,14 @@
+[tox]
+envlist = py27,
+          py35,
+          py36,
+          py37,
+minversion = 3.4.0
+skip_missing_interpreters = true
+
+[testenv]
+description = Run the unit tests under {basepython}
+skip_install = True
+usedevelop = True
+whitelist_externals = cargo
+commands = cargo test src/lib.rs