Since wendelin.core beginning PyBigFile.storeblk(blk, buf) is passed with
buf being buffer object. See 35eb95c2 (bigfile: Python wrapper around
virtual memory subsystem) for details about that. In its turn
ZBigFile.storeblk was initially making bytes copy snapshot of that
buffer, but later pushed that job to ZBlk.setblkdata in
e0b00bda (bigfile/zodb: Make "set blkdata to be later saved to DB"
explicit method of ZBlk). So on that codepath ZBlk.setblkdata(buf) is
invoked with buf being python buffer as argument name suggests.

However later, when working on WCFS, for preparing test data I also
explicitly used ZBlk.setblkdata in

    wcfs/internal/zdata/testdata/zblk_test_gen.py  2c152d41 (wcfs: Add zdata package to load ZBlk/ZBigFile data)
    wcfs/internal/xbtree/xbtreetest/treegen.py     a8595565 (wcfs: tests: Treegen functionality)

and there test data is prepared by regularly using ZODB without virtmem
layer involvement at all. ZBlk.setblkdata calls are made with bytes
and strings which happen to work by chance on py2 more or less but
turned out to break a bit on py3.

-> Prepare to make all that to work robustly by allowing ZBlk.setblkdata
to be called with buf being either buffer object, or any other object
that provides buffer interface.

Everything continues to work for py2 and we will fix issues around
treegen.py on py3 in the next patch.