[issue15806] Add context manager for the "try: ... except: pass" pattern

New submission from Raymond Hettinger:

It is a somewhat common pattern to write:

   try:
       do_something()
   except SomeException:
       pass

To search examples in the standard library (or any other code base) use:

    $ egrep -C2 "except( [A-Za-z]+)?:" *py  | grep -C2 "pass"

In the Python2.7 Lib directory alone, we find 213 examples.

I suggest a context manager be added that can ignore specifie exceptions.  Here's a possible implementation:

class Ignore:
    ''' Context manager to ignore particular exceptions'''

    def __init__(self, *ignored_exceptions):
        self.ignored_exceptions = ignored_exceptions

    def __enter__(self):
        return self

    def __exit__(self, exctype, excinst, exctb):
        return exctype in self.ignored_exceptions