The usage of `bytes` and `str` in the wcfs test was not consistent,
producing comparisons between `bytes` and `str` as well as errors such
as:

```python
___________________________________________________________________ test_wcfs_crash_old_data ___________________________________________________________________

    @func
    def test_wcfs_crash_old_data():
        # start wcfs with ΔFtail/ΔBtail not covering initial data.
        t = tDB(old_data=[{0:'a'}]); zf = t.zfile; at1 = t.head
        defer(t.close)

        f = t.open(zf)

        # ΔFtail coverage is currently (at1,at1]
        wl = t.openwatch()
        wl.watch(zf, at1, {})

        # wcfs was crashing on readPinWatcher -> ΔFtail.BlkRevAt with
        #   "at out of bounds: at: @at1,  (tail,head] = (@at1,@at1]
        # because BlkRevAt(at=tail) query was disallowed.
>       f.assertBlk(0, 'a')          # [0] becomes tracked

wcfs/wcfs_test.py:1844:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../venvs/wendelin.core/lib/python3.9/site-packages/decorator.py:232: in fun
    return caller(func, *(extras + args), **kw)
../pygolang/golang/__init__.py:125: in _
    return f(*argv, **kw)
wcfs/wcfs_test.py:721: in assertBlk
    t._assertBlk(blk, dataok, pinokByWLink)
../../venvs/wendelin.core/lib/python3.9/site-packages/decorator.py:232: in fun
    return caller(func, *(extras + args), **kw)
../pygolang/golang/__init__.py:125: in _
    return f(*argv, **kw)
wcfs/wcfs_test.py:810: in _assertBlk
    ev = doCheckingPin(_, pinokByWLink, pinfunc)
wcfs/wcfs_test.py:1091: in doCheckingPin
    wg.wait()
golang/_sync.pyx:246: in golang._sync.PyWorkGroup.wait
    ???
golang/_sync.pyx:226: in golang._sync.PyWorkGroup.go.pyrunf
    ???
wcfs/wcfs_test.py:1085: in _
    f(ctx, ev)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

ctx = <golang._context.PyContext object at 0x7fec13e76280>, ev = ['read pre']

    def _(ctx, ev):
        assert t.cached()[blk] == cached
        ev.append('read pre')

        # access data with released GIL so that the thread that reads data from
        # head/watch can receive pin message. Be careful to handle cancellation,
        # so that on error in another worker we don't get stuck and the
        # error can be propagated to wait and reported.
        #
        # we handle cancellation by spawning read in another thread and
        # waiting for either ctx cancel, or read thread to complete. This
        # way on ctx cancel (e.g. assertion failure in another worker), the
        # read thread can remain running even after _assertBlk returns, and
        # in particular till the point where the whole test is marked as
        # failed and shut down. But on test shutdown .fmmap is unmapped for
        # all opened tFiles, and so read will hit SIGSEGV. Prepare to catch
        # that SIGSEGV here.
        have_read = chan(1)
        def _():
            try:
                b = read_exfault_nogil(blkview[0:1])
            except SegmentationFault:
                b = 'FAULT'
            t._blkaccess(blk)
            have_read.send(b)
        go(_)
        _, _rx = select(
            ctx.done().recv,    # 0
            have_read.recv,     # 1
        )
        if _ == 0:
            raise ctx.err()
        b = _rx

>       ev.append('read ' + b)
E       TypeError: can only concatenate str (not "bytes") to str
```

I have changed `bytes` and `str` where necessary to make tests pass.