chore: make traceback module optional

some boards don't provide traceback so we can add a minimal implementation
that works _good enough_ in those cases so asyncio can still be used

asyncio/core.py

@@ -16,7 +16,12 @@
 """
 
 from adafruit_ticks import ticks_ms as ticks, ticks_diff, ticks_add
-import sys, select, traceback
+import sys, select
+
+try:
+    from traceback import print_exception
+except:
+    from .traceback import print_exception
 
 # Import TaskQueue and Task, preferring built-in C code over Python code
 try:
@@ -371,7 +376,7 @@ def default_exception_handler(loop, context):
         """The default exception handler that is called."""
 
         exc = context["exception"]
-        traceback.print_exception(None, exc, exc.__traceback__)
+        print_exception(None, exc, exc.__traceback__)
 
     def call_exception_handler(context):
         """Call the current exception handler. The argument *context* is passed through

asyncio/traceback.py

@@ -0,0 +1,57 @@
+# SPDX-FileCopyrightText: 2019-2020 Damien P. George
+#
+# SPDX-License-Identifier: MIT
+#
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019-2020 Damien P. George
+"""
+Fallback traceback module if the system traceback is missing.
+"""
+
+try:
+    from typing import List
+except ImportError:
+    pass
+
+import sys
+
+
+def _print_traceback(traceback, limit=None, file=sys.stderr) -> List[str]:
+    if limit is None:
+        if hasattr(sys, "tracebacklimit"):
+            limit = sys.tracebacklimit
+
+    n = 0
+    while traceback is not None:
+        frame = traceback.tb_frame
+        line_number = traceback.tb_lineno
+        frame_code = frame.f_code
+        filename = frame_code.co_filename
+        name = frame_code.co_name
+        print('  File "%s", line %d, in %s' % (filename, line_number, name), file=file)
+        traceback = traceback.tb_next
+        n = n + 1
+        if limit is not None and n >= limit:
+            break
+
+
+def print_exception(exception, value=None, tb=None, limit=None, file=sys.stderr):
+    """
+    Print exception information and stack trace to file.
+    """
+    if tb:
+        print("Traceback (most recent call last):", file=file)
+        _print_traceback(tb, limit=limit, file=file)
+
+    if isinstance(exception, BaseException):
+        exception_type = type(exception).__name__
+    elif hasattr(exception, "__name__"):
+        exception_type = exception.__name__
+    else:
+        exception_type = type(value).__name__
+
+    valuestr = str(value)
+    if value is None or not valuestr:
+        print(exception_type, file=file)
+    else:
+        print("%s: %s" % (str(exception_type), valuestr), file=file)