test: make check-py3-compat.py ignore empty code more reliably

It couldn't exclude an empty file containing comments. That's why
hgext/__init__.py had been listed in test-check-py3-compat.t before
"hgext: officially turn 'hgext' into a namespace package".

contrib/check-py3-compat.py

@@ -16,12 +16,12 b' def check_compat(f):'
     """Check Python 3 compatibility for a file."""
     with open(f, 'rb') as fh:
         content = fh.read()
+    root = ast.parse(content)
 
     # Ignore empty files.
-    if not content.strip():
+    if not root.body:
         return
 
-    root = ast.parse(content)
     futures = set()
     haveprint = False
     for node in ast.walk(root):