bpo-39406: os.putenv() avoids putenv_dict on Windows (GH-18126)

Windows: _wputenv(env) copies the *env* string and doesn't require
the caller to manage the variable memory.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index e0eecfa..71b99fd 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -819,7 +819,9 @@
     }
 }
 
-#ifdef HAVE_PUTENV
+/* Windows: _wputenv(env) copies the *env* string and doesn't require the
+   caller to manage the variable memory. */
+#if defined(HAVE_PUTENV) && !defined(MS_WINDOWS)
 #  define PY_PUTENV_DICT
 #endif
 
@@ -10130,8 +10132,10 @@
         posix_error();
         goto error;
     }
+    /* _wputenv(env) copies the *env* string and doesn't require the caller
+       to manage the variable memory. */
+    Py_DECREF(unicode);
 
-    posix_putenv_dict_setitem(name, unicode);
     Py_RETURN_NONE;
 
 error: