Based on the error traceback in the issue, I can see the problem. The issue is at line 760 in `_pytest/assertion/rewrite.py`:

```python
return "PYTEST_DONT_REWRITE" in docstring
```

The problem is that when the first expression in a Python file is a number (like `0`), it's being mistakenly treated as a docstring. When you try to use `in` operator on an integer, Python raises `TypeError: argument of type 'int' is not iterable`.

The fix needs to check if the docstring is actually a string before checking if it contains `"PYTEST_DONT_REWRITE"`.

Looking at the traceback:
- Line 695: `if self.is_rewrite_disabled(doc):`
- Line 760: `return "PYTEST_DONT_REWRITE" in docstring`

The `is_rewrite_disabled` method receives a `doc` parameter that could be an integer when the first expression is a number literal. The fix is to check if it's a string type first.

--- a/src/_pytest/assertion/rewrite.py
+++ b/src/_pytest/assertion/rewrite.py
@@ -757,4 +757,6 @@ class AssertionRewriter(ast.NodeVisitor):
 
     @staticmethod
     def is_rewrite_disabled(docstring: str) -> bool:
+        if not isinstance(docstring, str):
+            return False
         return "PYTEST_DONT_REWRITE" in docstring