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
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.

--------
kirr:

Add TODO to revert to import NEO globally after lab.nexedi.com/nexedi/neoppod/-/merge_requests/24 is landed.

/reviewed-by @kirr
/reviewed-on !27