[BUG] Database query expects mutable elements

[vojtsek]

Describe the bug
util/multiwoz/dbquery.py:42
The query() method expects elements of the constraints argument to be mutable.
However, tuples are used as the elements and given to the method call in the ConvLab code MultiWozEvaluator._final_goal_analyze().

To Reproduce
The error occurs when Analyzer.comprehensive_analysis() is called and the generated goal contains specific values, see call stack:

File "/lscratch/kulhajon/873944.stallo-adm.uit.no/source/train.py", line 214, in _run_evaluation
    result = analyzer(agent, num_dialogs=self.args.evaluation_dialogs)

  File "/lscratch/kulhajon/873944.stallo-adm.uit.no/source/evaluate.py", line 58, in __call__
    self.comprehensive_analyze(agent, agent.name, total_dialog=num_dialogs)

  File "/home/kulhajon/source/ConvLab-2/convlab2/util/analysis_tool/analyzer.py", line 145, in comprehensive_analyze
    task_success = sess.evaluator.task_success()

  File "/home/kulhajon/source/ConvLab-2/convlab2/evaluator/multiwoz_eval.py", line 298, in task_success
    goal_sess = self.final_goal_analyze()

  File "/home/kulhajon/source/ConvLab-2/convlab2/evaluator/multiwoz_eval.py", line 401, in final_goal_analyze
    match, mismatch = self._final_goal_analyze()

  File "/home/kulhajon/source/ConvLab-2/convlab2/evaluator/multiwoz_eval.py", line 382, in _final_goal_analyze
    query_result = self.database.query(domain, constraints)

  File "/home/kulhajon/source/ConvLab-2/convlab2/util/multiwoz/dbquery.py", line 34, in query
    ele[1] = 'centre'

TypeError: 'tuple' object does not support item assignment

Expected behavior
The method does not throw an error.

Additional context
It's reasonable to use immutable elements for other reasons too.
I've created the Pull Request with suggested fix.

[zqwerty]

thanks!