ctrl+c (SIGINT) stops the debugging with pyflyby but not with ipdb (PyInf#10417)

[dshivashankar1994]

ctrl+c stops the debugging with pyflyby but not with ipdb

Reproducer:

> cat /var/tmp/a.py
import os; print(os.getpid()); 
while True:
   import time
   time.sleep(8)
   break
p = 1 + "!@#"

> python /var/tmp/a.py &
 85791

> py -d 85791
Entering debugger.  Use 'n' to step, 'c' to continue running, 'q' to quit Python completely.
> /var/tmp/a.py(5)<module>()
      3    import time
      4    time.sleep(8)
----> 5    break
      6 p = 1 + "!@i#"
      7 raise Exception("A")

ipdb> ^C (**Ctrl+C**)
[email protected](hyd|codemill):~:> Exception ignored in: <generator object _DebuggerCtx at 0x7f39ea79ac00>
Traceback (most recent call last):
  File "/usr/local/python/python-3.10/std/lib64/python3.10/site-packages/pyflyby/_dbg.py", line 241, in _DebuggerCtx
  File "/opt/python/python-3.10/lib64/python3.10/contextlib.py", line 153, in __exit__
  File "/usr/local/python/python-3.10/std/lib64/python3.10/site-packages/pyflyby/_dbg.py", line 163, in _StdioCtx
  File "/opt/python/python-3.10/lib64/python3.10/contextlib.py", line 153, in __exit__
  File "/usr/local/python/python-3.10/std/lib64/python3.10/site-packages/pyflyby/_util.py", line 504, in nested
  File "/opt/python/python-3.10/lib64/python3.10/contextlib.py", line 576, in __exit__
  File "/opt/python/python-3.10/lib64/python3.10/contextlib.py", line 561, in __exit__
OSError: [Errno 5] Input/output error

with ipdb, this doesn't happen

> ipdb /var/tmp/a.py 
> /var/tmp/a.py(1)<module>()
----> 1 import os; print(os.getpid());
      2 while True:
      3    import time

ipdb> Ctrl+C --> No effect

Lets add a signal handler to make the behavior similar to ipdb debugger