py: Declare constant data as properly constant.

Otherwise some compilers (eg without optimisation) will put this read-only
data in RAM instead of ROM.
diff --git a/py/repl.c b/py/repl.c
index 4006bbf..997d800 100644
--- a/py/repl.c
+++ b/py/repl.c
@@ -223,7 +223,7 @@
                 // If there're no better alternatives, and if it's first word
                 // in the line, try to complete "import".
                 if (s_start == org_str) {
-                    static char import_str[] = "import ";
+                    static const char import_str[] = "import ";
                     if (memcmp(s_start, import_str, s_len) == 0) {
                         *compl_str = import_str + s_len;
                         return sizeof(import_str) - 1 - s_len;