Use list for compose_files instead of map

This variable gets read twice when loading projects, so we need to
use a list rather than a map so the value doesn't get blown away.

Author: Secretions

src/pytest_docker_compose/__init__.py

@@ -126,7 +126,7 @@ def docker_project(self, request):
             project_dir = '.'
 
         # py35 needs strings for os.path functions
-        compose_files = map(str, compose_files)
+        compose_files = [str(p) for p in compose_files]
 
         project = project_from_options(
             project_dir=str(project_dir),