Some tests fail on Marshmallow validation errors

[Flix6x]

Specifically, I got:

=========================== short test summary info ============================
ERROR flexmeasures/data/schemas/reporting/__init__.py - TypeError: Field.__init__() got an unexpected keyword argument 'validator'
ERROR flexmeasures/data/schemas/reporting/aggregation.py - TypeError: Field.__init__() got an unexpected keyword argument 'validator'
ERROR flexmeasures/data/schemas/reporting/pandas_reporter.py - TypeError: Field.__init__() got an unexpected keyword argument 'validator'
ERROR flexmeasures/data/schemas/reporting/profit.py - TypeError: Field.__init__() got an unexpected keyword argument 'validator'
ERROR flexmeasures/data/schemas/scheduling/__init__.py - TypeError: Field.__init__() got an unexpected keyword argument 'default'
ERROR flexmeasures/data/schemas/scheduling/process.py - TypeError: Field.__init__() got an unexpected keyword argument 'default'
ERROR flexmeasures/data/schemas/scheduling/storage.py - TypeError: Field.__init__() got an unexpected keyword argument 'default'
!!!!!!!!!!!!!!!!!!! Interrupted: 7 errors during collection !!!!!!!!!!!!!!!!!!!!
======================== 79 warnings, 7 errors in 0.57s ========================

which I believe is the same issue as marshmallow-code/marshmallow#2820. It seems we have some work to upgrade to Marshmallow 4.