monotone_constraints is expected to be a str, rather than tuple

[wangminzheng]

In the official document, the value of monotone_constraints is a tuple:
params_constrained['monotone_constraints'] = (1,-1)
See: https://xgboost.readthedocs.io/en/stable/tutorials/monotonic.html

But in the source code, it's evaluated to be either a str or a dict:
if isinstance(value, str):
return value
constrained_features = set(value.keys())
See:

xgboost/python-package/xgboost/core.py

Line 1413 in 94ca52b

constrained_features = set(value.keys())

When passing the argument as a tuple, it will throw an error say tuple doesn't have attribute "keys"