lint: fix failing code check

8d2b62d716b0 introduced code that `test-check-code.t` wasn't happy about, this
fixes it.

Differential Revision: https://phab.mercurial-scm.org/D10506

setup.py

@@ -438,7 +438,8 b" elif os.path.exists('.hg_archival.txt'):"
     else:
         version = '0+hg' + kw.get('node', '')[:12]
 elif os.path.exists('mercurial/__version__.py'):
-    data = open('mercurial/__version__.py').read()
+    with open('mercurial/__version__.py') as f:
+        data = f.read()
     version = re.search('version = b"(.*)"', data).group(1)
 
 if version: