setup: show how to set the module policy for imports

This is not technically needed, since mercurial.__version__
does not exist as a native module, but, without this style wrappings,
if something else had a native flavor, the module loader would get
upset.

In principle, the `env` object is trying to set HGMODULEPOLICY for
children, so, conceptually we should set it for this in-process
child.

setup.py

@@ -212,10 +212,17 b' if version:'
         f.write('version = "%s"\n' % version)
 
 try:
+    oldpolicy = os.environ.get('HGMODULEPOLICY', None)
+    os.environ['HGMODULEPOLICY'] = 'py'
     from mercurial import __version__
     version = __version__.version
 except ImportError:
     version = 'unknown'
+finally:
+    if oldpolicy is None:
+        del os.environ['HGMODULEPOLICY']
+    else:
+        os.environ['HGMODULEPOLICY'] = oldpolicy
 
 class hgbuild(build):
     # Insert hgbuildmo first so that files in mercurial/locale/ are found