Issue2056 - Prevent tracebacks from accumulating in REPL

[DavePutz]

A minor bug fix for issue micropython#2056. Clear the exception traceback after printing in REPL.

[tannewt]

Thank you @DavePutz!

[tannewt]

Build failure was a network issue.

[cwalther]

This is wrong. It prevents printing an exception twice, which I do in #3696 (once to measure the length of the traceback, once to get the text) and which people may want to do for other reasons, it’s available from Python via sys.print_exception(). (For some reason, it also causes the terminalio screen to be drawn over the stage screen, haven’t investigated that yet.)

The problem observed in micropython#2056 is specific to adafruit_midi in that it raises the same exception instance every time, a usage that apparently wasn’t anticipated in the core. Will have to dig deeper into how exceptions work to figure out how to properly fix that. It may be nontrivial – there would need to be a way to distinguish an exception reused in the way adafruit_midi does it from one re-raised in an except block.

In fact, I’m not sure it should be fixed, since CPython appears to do the same:

Python 3.8.3 (default, Jul  8 2020, 14:25:02) 
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> e = ValueError('X')
>>> raise e
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    <?xml version="1.0" encoding="UTF-8"?>
ValueError: X
>>> raise e
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    <?xml version="1.0" encoding="UTF-8"?>
  File "<stdin>", line 1, in <module>
    <?xml version="1.0" encoding="UTF-8"?>
ValueError: X
>>> raise e
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    <?xml version="1.0" encoding="UTF-8"?>
  File "<stdin>", line 1, in <module>
    <?xml version="1.0" encoding="UTF-8"?>
  File "<stdin>", line 1, in <module>
    <?xml version="1.0" encoding="UTF-8"?>
ValueError: X

So maybe the proper fix is rather to make adafruit_midi stop reusing exceptions.