core/stats: get rid of duplicated keys for 'auto stats'

previously:
```
{'iter_data': {'iter_data': {'count': 9, 'last': datetime.datetime(2020, 1, 3, 1, 1, 1)}}}
```

after
```
{'iter_data': {'count': 9, 'last': datetime.datetime(2020, 1, 3, 1, 1, 1)}}
```

my/core/tests/auto_stats.py

@@ -0,0 +1,30 @@
+"""
+Helper 'module' for test_guess_stats
+"""
+
+from dataclasses import dataclass
+from datetime import datetime, timedelta
+from pathlib import Path
+from typing import Iterable, Sequence
+
+
+@dataclass
+class Item:
+    id: str
+    dt: datetime
+    source: Path
+
+
+def inputs() -> Sequence[Path]:
+    return [
+        Path('file1.json'),
+        Path('file2.json'),
+        Path('file3.json'),
+    ]
+
+
+def iter_data() -> Iterable[Item]:
+    dt = datetime.fromisoformat('2020-01-01 01:01:01')
+    for path in inputs():
+        for i in range(3):
+            yield Item(id=str(i), dt=dt + timedelta(days=i), source=path)