py/repl: Skip private variables when printing tab completion options.

Any '_' variables/functions in frozen modules are currently printed, when
they shouldn't be.  That's due to underscore names possibly existing
between the start and end qstrs which are used to print the auto-complete
matches.  The underscore names should be skipped when iterating between the
two boundary qstrs.

The underscore attributes are removed from the extra coverage exp file
because tab completing "import <tab>" no longer lists modules beginning
with an underscore.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

py/repl.c

@@ -218,6 +218,10 @@ static void print_completions(const mp_print_t *print,
     for (qstr q = q_first; q <= q_last; ++q) {
         size_t d_len;
         const char *d_str = (const char *)qstr_data(q, &d_len);
+        // filter out words that begin with underscore unless there's already a partial match
+        if (s_len == 0 && d_str[0] == '_') {
+            continue;
+        }
         if (s_len <= d_len && strncmp(s_start, d_str, s_len) == 0) {
             if (test_qstr(obj, q)) {
                 int gap = (line_len + WORD_SLOT_LEN - 1) / WORD_SLOT_LEN * WORD_SLOT_LEN - line_len;