I have assembled a patch that adds some features from
my own development path of tarfile.py
(http://www.gustaebel.de/lars/tarfile/) and fixes
#1471427 which made some restructuring necessary. The
patch affects Lib/tarfile.py, Lib/test/test_tarfile.py
and Doc/lib/libtarfile.tex.

The changes the patch makes are as follows:

Sets the version to 0.8.0.

Support for base256 encoding of number fields (nti()
and itn()). Up to now this was hardcoded for the
filesize field to allow filesizes greater than 8 GB but
it is applicable to all number fields.

TarInfo.tobuf() has a boolean argument "posix" which
controls how number fields are written (base256 is
non-posix).

Both unsigned and signed (Sun and NeXT) checksums are
calculated. Header validation moves from TarFile.next()
to TarInfo.frombuf(). A header is valid only if its
checksum is okay, in the past the checksum was
calculated but ignored.

The TarFile.next() method was rearranged which makes
header processing clearer and more abstract and fixes
bug #1471427. However, this change breaks the interface
for subclassing in order to implement custom member
types but makes it much easier at the same time. The
mapping TYPE_METH was removed.

A new test ReadGNULongTest was added to test_tarfile.py
and testtar.tar was updated to be able to test the GNU
extensions LONGNAME and LONGLINK.