Maybe you have your solution already. but it is related to tuples you need to use:

For a many2many field, a list of tuples is expected. Following are the list of how to pass tuples.

(0, 0, { values }) link to a new record that needs to be created with the given values dictionary

(1, ID, { values }) update the linked record with id = ID (write values on it)

(2, ID) remove and delete the linked record with id = ID (calls unlink on ID, that will delete the object completely, and the link to it as well)

(3, ID) cut the link to the linked record with id = ID (delete the relationship between the two objects but does not delete the target object itself)

(4, ID) link to existing record with id = ID (adds a relationship)

(5) unlink all (like using (3,ID) for all linked records)

(6, 0, [IDs]) replace the list of linked IDs (like using (5) then (4,ID) for each ID in the list of IDs)

with that syntax it should work, and it will add your tag: 

'category_id':[(6,0,[21])]