If a tarball has a-x perms set on its root directory, one cannot access
its contents. 

$ tar zxf generator_tools-0.3.5.tar.gz.
$ ls generator_tools-0.3.5/
ls: cannot access generator_tools-0.3.5/README.txt: Permission denied
...
sridharr@double:/tmp/i$ 

This is fine for GNU tar (the user can always do a chmod +x later). But
for the tarfile library, it would be better to have a flag such as
readaccess=True that will force ``extractall`` to enforce *minimum*
permissions required for the basic read access. This means, tarfile
would ignore u-x on directories and u-r on files.

The reason I make this feature request (instead of working around the
issue myself in a verbose way) is that the very reason to write a
program to extract tarball (instead of doing it manually) is to automate
it .. which automation is more effective and simple if ``extractall``
had a flag such as readaccess=True.