Check if assignment target is a name when finding version

Closes gh-343

flit_core/flit_core/common.py

@@ -128,10 +128,13 @@ def get_docstring_and_version_via_ast(target):
     for child in node.body:
         # Only use the version from the given module if it's a simple
         # string assignment to __version__
-        is_version_str = (isinstance(child, ast.Assign) and
-                          len(child.targets) == 1 and
-                          child.targets[0].id == "__version__" and
-                          isinstance(child.value, ast.Str))
+        is_version_str = (
+                isinstance(child, ast.Assign)
+                and len(child.targets) == 1
+                and isinstance(child.targets[0], ast.Name)
+                and child.targets[0].id == "__version__"
+                and isinstance(child.value, ast.Str)
+        )
         if is_version_str:
             version = child.value.s
             break