Fix backward compatibility

The previous solution was breaking backward compatibility when for
instance `docken_compose_file` was returning a `py.path` path (used by
pytest internally, e.g. for `pytestconfig.rootdir`).

This makes it more conservative and more backward compatible.

src/pytest_docker/__init__.py

@@ -102,11 +102,10 @@ def wait_until_responsive(self, check, timeout, pause,
         )
 
 
-def str_to_list(x):
-    if isinstance(x, str):
-        return [x]
-    else:
-        return x
+def str_to_list(arg):
+    if isinstance(arg, (list, tuple)):
+        return arg
+    return [arg]
 
 
 @attr.s(frozen=True)

tests/test_dockercomposeexecutor.py

@@ -2,6 +2,8 @@
 import mock
 import subprocess
 
+import py
+
 from pytest_docker import DockerComposeExecutor
 
 
@@ -17,6 +19,20 @@ def test_execute():
         ]
 
 
+def test_pypath_compose_files():
+    compose_file = py.path.local('/tmp/docker-compose.yml')
+    docker_compose = DockerComposeExecutor(compose_file, "pytest123")
+    with mock.patch('subprocess.check_output') as check_output:
+        docker_compose.execute("up")
+        assert check_output.call_args_list == [
+            mock.call(
+                'docker-compose -f "/tmp/docker-compose.yml"'
+                ' -p "pytest123" up',
+                shell=True, stderr=subprocess.STDOUT,
+            ),
+        ]
+
+
 def test_multiple_compose_files():
     docker_compose = DockerComposeExecutor(
         ["docker-compose.yml", "other-compose.yml"], "pytest123")