zodbdump: Fix pickle disassembly on py3

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"

zodbtools/zodbdump.py

@@ -76,6 +76,7 @@ import logging as log
 import re
 from golang.gcompat import qq
 from golang import func, defer, strconv, b
+from six import StringIO  # io.StringIO does not accept non-unicode strings on py2
 
 # txn_raw_extension returns raw extension from txn metadata
 def txn_raw_extension(stor, txn):
@@ -122,9 +123,9 @@ def zodbdump(stor, tidmin, tidmax, hashonly=False, pretty='raw', out=asbinstream
             else:
                 out.write(b"extension\n")
                 extf = BytesIO(rawext)
-                disf = BytesIO()
+                disf = StringIO()
                 pickletools.dis(extf, disf)
-                out.write(indent(disf.getvalue(), "  "))
+                out.write(b(indent(disf.getvalue(), "  ")))
                 extra = extf.read()
                 if len(extra) > 0:
                     out.write(b"  + extra data %s\n" % qq(extra))
@@ -161,10 +162,10 @@ def zodbdump(stor, tidmin, tidmax, hashonly=False, pretty='raw', out=asbinstream
                     elif pretty == 'zpickledis':
                         # https://github.com/zopefoundation/ZODB/blob/5.6.0-55-g1226c9d35/src/ZODB/serialize.py#L24-L29
                         dataf = BytesIO(obj.data)
-                        disf  = BytesIO()
+                        disf  = StringIO()
                         pickletools.dis(dataf, disf) # class
                         pickletools.dis(dataf, disf) # state
-                        out.write(indent(disf.getvalue(), "  "))
+                        out.write(b(indent(disf.getvalue(), "  ")))
                         extra = dataf.read()
                         if len(extra) > 0:
                             out.write(b"  + extra data %s\n" % qq(extra))