Fix sig_block() doctest

[roed]

The following problem arose while working on #12415. Put the following code in your ~/.sage/init.sage:

import multiprocessing
class Task(multiprocessing.Process):
    def run(self):
        sage_namespace = dict(sage.all_cmdline.__dict__)
        exec compile("from sage.tests.interrupt import *","","single",0,1) in sage_namespace
        exec compile("test_sig_block()","","single",0,1) in sage_namespace
        exec compile("sig_on_count()","","single",0,1) in sage_namespace
Task().start()

It will result in the following being printed (though your Sage process survives since the segfault occurs in a chile process):

42
0

------------------------------------------------------------------------
Unhandled SIGSEGV: A segmentation fault occurred in Sage.
This probably occurred because a *compiled* component of Sage has a bug
in it and is not properly wrapped with sig_on(), sig_off(). You might
want to run Sage under gdb with 'sage -gdb' to debug this.
Sage will now terminate.
------------------------------------------------------------------------

Typing it in to a running Sage session does not cause the same failure.

[jdemeyer]

I found the culprit. Putting all the code inside try:/except: instead of just sig_on() fixes the problem.

[jdemeyer]

Apparently, it is illegal to exit a try block while sig_on() is in effect.

Patch (which also adds some debug code for sig_block()/sig_unblock()) needs review.

[vbraun]

I suppose that it does confuse something on the Python heap when you suddenly jump back into the try/except block?

Patch looks good to me.