pickletools.dis, which is used to handle --pretty=zpickledis (*),
expects output stream be text-like, not binary. We were passing a binary
stream to it. As the result pickle disassembly was failing on py3:

    _______________________ test_zodbdump[!zext-zpickledis] ________________________

    tmpdir = local('/tmp/pytest-of-kirr/pytest-11/test_zodbdump__zext_zpickledis0')
    zext = <function zext.<locals>._ at 0x7f538b508670>, pretty = 'zpickledis'

        @mark.parametrize('pretty', ('raw', 'zpickledis'))
        def test_zodbdump(tmpdir, zext, pretty):
            tdir  = dirname(__file__)
            zkind = '_!zext' if zext.disabled else ''
            tfs1  = fs1_testdata_py23(tmpdir, '%s/testdata/1%s.fs' % (tdir, zkind))
            stor  = FileStorage(tfs1, read_only=True)

            with open('%s/testdata/1%s.zdump.%s.ok' % (tdir, zkind, pretty), 'rb') as f:
                dumpok = f.read()

            out = BytesIO()
    >       zodbdump(stor, None, None, pretty=pretty, out=out)

    zodbtools/test/test_dump.py:48:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    zodbtools/zodbdump.py:165: in zodbdump
        pickletools.dis(dataf, disf) # class
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    pickle = <_io.BytesIO object at 0x7f538b577130>
    out = <_io.BytesIO object at 0x7f538b49f8b0>, memo = {}, indentlevel = 4
    annotate = 0

        def dis(pickle, out=None, memo=None, indentlevel=4, annotate=0):
            """Produce a symbolic disassembly of a pickle..."""
            ...
            for opcode, arg, pos in genops(pickle):
                if pos is not None:
    >               print("%5d:" % pos, end=' ', file=out)
    E               TypeError: a bytes-like object is required, not 'str'

    /usr/lib/python3.9/pickletools.py:2450: TypeError

-> Fix it by letting pickletools.dis to emit its output to StringIO instead of BytesIO.

(*) see 80559a94 "zodbdump: support --pretty option with a format to show
    pickles disassembly"