Bug #50461: fix pep8 E711 and E712

Bug #44222: Fix overflowerror when system time changes

Bug #44222: Fix overflowerror when system time changes

Bug #47114: Fix memory leak - traceback

The Python garbage collector (GC) nowadays can free cyclic structure,
but keeping a Traceback object is still considered a bad idea: It
contains a reference to all stack frames, which reference all local
variables, and thus the traceback itself.

Format the traceback once and replace the reference with None.
The standard traceback library handles this fine.

<https://docs.python.org/2/library/sys.html#sys.exc_info>

Fix KeyError if socket is removed in timer

The following script demonstrates a KeyError which is caused if a socket which currently has some pending input/output was removed in a notifier timer.
The problem is that step() first poll()s, then executes the timers() and then uses the polled FD's.

$ cat timer_bug.py
import notifier
import subprocess
notifier.init()

fd = None
def on_read(sock):
        print 'on_read'
        return True
def on_timer():
        print 'on_timer'
        notifier.socket_remove(fd)
        return True
p = subprocess.Popen('python -c "import sys; sys.stdin.read()"', shell=True, stdin=subprocess.PIPE)
fd = p.stdin.fileno()
notifier.socket_add(fd, on_read)
notifier.timer_add(0, on_timer)
p.communicate('A')
notifier.step()

$ python timer_bug.py
on_timer
Traceback (most recent call last):
  File "timer_bug.py", line 18, in <module>
    notifier.step()
  File "/home/git/python-notifier/notifier/nf_generic.py", line 259, in step
    sock_obj = __sock_objects[ fd ]
KeyError: 4

applied patch from Sönke Schwardt for Qt notifier and set version to …

…0.9.7