Warning: The 'missing' attribute of fields is deprecated

[tiltowait]

When I run unit tests involving umongo, I get the following warning message (actually, if I run the tests via pytest instead of unittest, I get hundreds of the same warning):

/Users/tiltowait/Library/Caches/pypoetry/virtualenvs/inconnu-qKm_l4La-py3.10/lib/python3.10/site-packages/umongo/data_proxy.py:160: RemovedInMarshmallow4Warning: The 'missing' attribute of fields is deprecated. Use 'load_default' instead.
   if callable(field.missing):
 /Users/tiltowait/Library/Caches/pypoetry/virtualenvs/inconnu-qKm_l4La-py3.10/lib/python3.10/site-packages/umongo/data_proxy.py:163: RemovedInMarshmallow4Warning: The 'missing' attribute of fields is deprecated. Use 'load_default' instead.
   self._data[mongo_name] = field.missing
 /Users/tiltowait/Library/Caches/pypoetry/virtualenvs/inconnu-qKm_l4La-py3.10/lib/python3.10/site-packages/umongo/data_proxy.py:161: RemovedInMarshmallow4Warning: The 'missing' attribute of fields is deprecated. Use 'load_default' instead.
   self._data[mongo_name] = field.missing()

Is there any plan to address this? It looks like a simple fix, but I'm not familiar with umongo's code to be confident in that assessment.

[lafrech]

The change shouldn't be difficult. Just replace missing with load_default in the right place(s).

The trick is that load_default was introduced in marshmallow 3.13 and umongo supports marshmallow 3.10. If we use missing in a few places, we can make it conditional (use the right attribute name depending on marshmallow version). Otherwise, we may be tempted to drop marshmallow < 3.13.

Would you like to try and send a PR?