Fix how unpatched ZODB4 is reported to lack required patch:

Before:

    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/wendelin.core/lib/tests/test_zodb.py", line 251, in test_zconn_at
        assert zconn_at(conn1) == at0
      File "/home/kirr/src/wendelin/wendelin.core/lib/zodb.py", line 162, in zconn_at
        assert 'conn:MVCC-via-loadBefore-only' in ZODB.nxd_patches, \
    AttributeError: 'module' object has no attribute 'nxd_patches'

After:

    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/wendelin.core/lib/tests/test_zodb.py", line 251, in test_zconn_at
        assert zconn_at(conn1) == at0
      File "/home/kirr/src/wendelin/wendelin.core/lib/zodb.py", line 163, in zconn_at
        "nexedi/ZODB!1")
      File "/home/kirr/src/wendelin/wendelin.core/lib/zodb.py", line 191, in _zassertHasNXDPatch
        (zmajor, patch, details_link))
    AssertionError: ZODB4 is not patched with required Nexedi patch 'conn:MVCC-via-loadBefore-only'
            See nexedi/ZODB!1 for details

Fixes 1f866c00 (lib/zodb: Teach zconn_at to work on ZODB4).