git/packwriter: open(..) prohibited in __del__

git/packwriter: open(..) prohibited in __del__

When an exception occurs, __del__ is invoked by the interpreter, to
perform cleanup. It seems that since Python 3.4, the behaviour has
changed, and also prohibits invocations of open(..) (source:
https://stackoverflow.com/a/29737870). Instead, contextmanager API
should be used (source: https://stackoverflow.com/a/26544629), which
seems to be in place already.

This should fix exception messages such as 'NameError: name 'open'
is not defined'.

Signed-off-by: Bas Stottelaar <basstottelaar@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>