patch 8.1.0888: the a: dict is not immutable as documented

Problem:    The a: dict is not immutable as documented.
Solution:   Make the a:dict immutable, add a test. (Ozaki Kiichi, Yasuhiro
            Matsumoto, closes #3929)
diff --git a/src/version.c b/src/version.c
index 94825f8..e31b7ca 100644
--- a/src/version.c
+++ b/src/version.c
@@ -784,6 +784,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    888,
+/**/
     887,
 /**/
     886,