setup: fix localhgenv

It should return env as a dict instead of None.

setup.py

@@ -227,6 +227,7 b' def localhgenv():'
         # SystemRoot is required by Windows to load various DLLs.  See:
         # https://bugs.python.org/issue13524#msg148850
         env['SystemRoot'] = os.environ['SystemRoot']
+    return env
 
 version = ''