Try to squash struct.pack warnings on the "amd64 gentoo trunk"

Try to squash struct.pack warnings on the "amd64 gentoo trunk"
buildbot (& possibly other 64-bit boxes) during test_gzip.

The native zlib crc32 function returns an unsigned 32-bit integer,
which the Python wrapper implicitly casts to C long.  Therefore the
same crc can "look negative" on a 32-bit box but "look positive" on
a 64-bit box.  This patch papers over that platform difference when
writing the crc to file.

It may be better to change the Python wrapper, either to make
the result "look positive" on all platforms (which means it may
have to return a Python long at times on a 32-bit box), or to
keep the sign the same across boxes.  But that would be a visible
change in what users see, while the current hack changes no
visible behavior (well, apart from stopping the struct deprecation
warning).

Note that the module-level write32() function is no longer used.