positive_id was added in 2004 in f7b96aea (ZODB.utils grows a new
function positive_id(), which returns the id() of an object as a
non-negative integer ...) to workaround change in behaviour Python2.3 ->
2.4 for `'%x' % <negative-number>`. And nowdays `positive_id` is used
only in one place in Connection.__repr__ to mimic approximately what
Python already gives for a default __repr__ out of the box.

On https://github.com/zopefoundation/ZODB/actions/runs/5067242751/jobs/9098062772
we recently hit

    ZODB.POSException.InvalidObjectReference: <unprintable InvalidObjectReference object>

for the following code:

    if self._jar.get_connection(database_name) is not obj._p_jar:
        raise InvalidObjectReference(
            "Attempt to store a reference to an object from "
            "a separate connection to the same database or "
            "multidatabase", self._jar, obj,
        )

    ( https://github.com/zopefoundation/ZODB/blob/5.8.0-24-g8a7e3162d/src/ZODB/serialize.py#L366-L371 )

for which the "unprintable ..." part is produced by `Lib/traceback.py`
who tries to do `str(ex)` and returns `unprintable ...` if that str
failed:

    https://github.com/python/cpython/blob/v3.8.16-18-g9f89c471fb1/Lib/traceback.py#L153-L157

Since one of the reasons for that failure might have been a bit
non-trivial code and assert in positive_id, let's remove it and rely on
builtin python behaviour for Connection repr.

repr for Connection:

before this patch:

    <Connection at 0x7fef09989d10>

after this patch:

    <ZODB.Connection.Connection object at 0x7fef09989d10>

/reviewed-by @dataflake, @d-maurer 
/reviewed-at https://github.com/zopefoundation/ZODB/pull/384