PY_CFLAGS doesn't exist under Windows

Lib/test/support.py

@@ -890,7 +890,7 @@ def gc_collect():
 
 def python_is_optimized():
     """Find if Python was built with optimizations."""
-    cflags = sysconfig.get_config_vars()['PY_CFLAGS']
+    cflags = sysconfig.get_config_var('PY_CFLAGS') or ''
     final_opt = ""
     for opt in cflags.split():
         if opt.startswith('-O'):