Only display exception context if __suppress_context__ not set

[tpoliaw]

The built-in repl only displays the context of an exception (the "During
the handling of the above exception..." message) if suppress_context
is not set, eg by using

raise Exception() from xyz

This makes ptpython behave the same way.

See description of default behaviour in the python docs:

https://docs.python.org/3/library/exceptions.html#BaseException.__suppress_context__

[tpoliaw]

Example to demonstrate the behaviour

try:
    1/0
except ZeroDivisionError:
    raise ValueError("zero") from None