format tracebacks with cause

[dneuhaeuser-zalando]

closes #99

Here is a script that shows how this changes the traceback formatting:

import sys
import traceback


try:
    try:
        raise ValueError("some value")
    except ValueError:
        raise TypeError("some type")
except Exception:
    exc_value, exc_type, exc_tb = sys.exc_info()
    print("".join(traceback.format_tb(exc_tb)))
    print("=" * 80)
    print("".join(traceback.format_exception(None, exc_type, exc_tb)))
    print("=" * 80)
    print("".join(traceback.format_exception(exc_value, None, exc_tb)))
    print("=" * 80)
    print("".join(traceback.format_exception(exc_value, exc_type, exc_tb)))

Output:

$ python foo.py 
  File "foo.py", line 9, in <module>
    raise TypeError("some type")

================================================================================
Traceback (most recent call last):
  File "foo.py", line 7, in <module>
    raise ValueError("some value")
ValueError: some value

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "foo.py", line 9, in <module>
    raise TypeError("some type")
TypeError: some type

================================================================================
Traceback (most recent call last):
  File "foo.py", line 9, in <module>
    raise TypeError("some type")
NoneType: None

================================================================================
Traceback (most recent call last):
  File "foo.py", line 7, in <module>
    raise ValueError("some value")
ValueError: some value

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "foo.py", line 9, in <module>
    raise TypeError("some type")
TypeError: some type

[codeboten]

Thanks for the PR! This looks good to me.