WIP Fix gdb debugging and test it on Travis CI

[jdemeyer]

The cysignals-CSI is currently broken for different reasons:

• The Python library which is being debugged and the Python runtime started by GDB may mismatch.
• There are various bytes vs. str incompatibilities in the script.
• Cython debugging does not work with Python 3.4 due to Cython/Debugger/libpython.py does not support PEP 393 cython/cython#1655

[anntzer]

Also, would it be possible to have a way to just disable gdb debugging? I mostly just want to use cysignals', well, signal handling facility, and only work on Py3, so at that point the tracebacks generated by gdb failures are just noise to me.

[jdemeyer]

Also, would it be possible to have a way to just disable gdb debugging?

What do you have in mind? A compile-time setting, run-time setting, environment variable...?

[anntzer]

I think gdb integration should be turned on either by an environment variable (my preference, as it is the easiest to set without modifying the source code) or by a runtime function (e.g. cysignals.run_gdb_on_failure()). Or possibly even by a C macro, e.g. NDEBUG (not sure I really like that idea).

[collares]

testgdb.py says it "is known to be broken in some cases, see [this issue]", but this issue was closed. Does this mean previously-known issues were fixed, or just are no longer relevant?