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

Any '_' variables/functions in frozen modules are currently printed.

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

Author: pi-anl

tests/cmdline/repl_autocomplete_underscore.py

@@ -0,0 +1,11 @@
+# Test REPL autocompletion for underscore attributes in frozen modules
+
+try:
+    import asyncio
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+# Simulate typing 'asyncio.' and pressing tab
+# The tab character after `asyncio.` below triggers the completion
+asyncio.	t	

tests/cmdline/repl_autocomplete_underscore.py.exp

@@ -0,0 +1,26 @@
+MicroPython v1.25.0-preview.532.g8c012467d7 on 2025-04-10; linux [GCC 14.2.1] version
+Use Ctrl-D to exit, Ctrl-E for paste mode
+>>> # Test REPL autocompletion for underscore attributes in frozen modules
+>>> 
+>>> try:
+...        import asyncio
+... except ImportError:
+...     print("SKIP")
+...     raise SystemExit
+... 
+>>> # Simulate typing 'asyncio.' and pressing tab
+>>> # The tab character after `asyncio.` below triggers the completion
+>>> asyncio.
+CancelledError  Task            TaskQueue       __dict__
+__file__        __path__        cur_task        lock
+select          sleep           sleep_ms        sys
+ticks_add       ticks_diff      IOQueue         Lock
+Loop            SingletonGenerator              StreamReader
+StreamWriter    TimeoutError    __version__     core
+create_task     current_task    funcs           gather
+get_event_loop  new_event_loop  open_connection
+run             run_until_complete              start_server
+stream          ticks           wait_for        wait_for_ms
+>>> asyncio.ticks
+<function>
+>>>