Do not import NEO globally.

NEO is still not ported to Python 3.
Importing NEO globally thus makes pytest tests fail during the
assert-rewritting step:
```python
../../venvs/wendelin.core/lib/python3.9/site-packages/_pytest/assertion/rewrite.py:178: in exec_module
    exec(co, module.__dict__)
lib/tests/test_zodb.py:36: in <module>
    from neo.client.Storage import Storage as NEOStorage
../neoppod/neo/client/__init__.py:52: in <module>
    from . import app # set up signal handlers early enough to do it in the main thread
E     File "/srv/slapgrid/slappart66/git/neoppod/neo/client/app.py", line 356
E       except NEOStorageReadRetry, e:
E                                 ^
E   SyntaxError: invalid syntax
```

A MR adding enough support to not fail at import time is proposed in
nexedi/neoppod!24 .
However, that MR will not be reviewed until the vacation period is over.

In the meantime, and as a previous step to make running NEO tests
optional, the import has been moved inside the function loading NEO.
Thus, only the tests that require NEO will fail.

lib/tests/test_zodb.py

@@ -33,7 +33,6 @@ from golang import defer, func
 from pytest import raises
 import pytest; xfail = pytest.mark.xfail
 from ZEO.ClientStorage import ClientStorage as ZEOStorage
-from neo.client.Storage import Storage as NEOStorage
 import os
 from six.moves.urllib.parse import quote_plus
 
@@ -494,6 +493,7 @@ def test_zstor_2zurl(tmpdir, neo_ssl_dict):
     # neo returns new NEO client for specified cluster name and master address.
     # NOTE, similarly to ZEO, the client is returned without waiting until server nodes are connected.
     def neo(cluster_name, master_addr, ssl=0):
+        from neo.client.Storage import Storage as NEOStorage
         kwargs = dict(master_nodes=master_addr, name=cluster_name)
         if ssl:
             kwargs.update(neo_ssl_dict)