py, unix: Allow to compile with -Wsign-compare.

See issue #699.
diff --git a/py/lexer.c b/py/lexer.c
index f4aedb4..13c3a3e 100644
--- a/py/lexer.c
+++ b/py/lexer.c
@@ -657,7 +657,7 @@
         // need to check for this special token in many places in the compiler.
         // TODO improve speed of these string comparisons
         //for (mp_int_t i = 0; tok_kw[i] != NULL; i++) {
-        for (mp_int_t i = 0; i < MP_ARRAY_SIZE(tok_kw); i++) {
+        for (size_t i = 0; i < MP_ARRAY_SIZE(tok_kw); i++) {
             if (str_strn_equal(tok_kw[i], lex->vstr.buf, lex->vstr.len)) {
                 if (i == MP_ARRAY_SIZE(tok_kw) - 1) {
                     // tok_kw[MP_ARRAY_SIZE(tok_kw) - 1] == "__debug__"