Python 3 preparation: Change syntax of raise with traceback

[wluebbe]

2to3 proposes changes for

raise E, V, T -> raise E(V).with_traceback(T)
raise E, None, T -> raise E.with_traceback(T)

These changes are invalid (see also #18796).

Several options are given in the documentation of python-future (see ​http://python-future.org/compatible_idioms.html#raising-exceptions).

This ticket is tracked as a dependency of meta-ticket #16052. The simple raise statements are fixed in #15990.

[wluebbe]

New commits:

​249b939

Trac #18799: Change syntax of raise with traceback

[chapoton]

I propose to convert the elliptic curve one to a simple raise (no traceback)

[chapoton]

ok, let it be.