Fix tuple input types in base contract

[kongzii]

What was wrong?

argument_types in the base contract were initialised incorrectly for tuples.

If you revert the changes in the base_contract.py, then the newly added tests will fail on the following error:

E               AssertionError: assert ('tuple',) == ('(uint256,ui...dress[])[])',)
E                 
E                 At index 0 diff: 'tuple' != '(uint256,uint256[],(int256,bool[2],address[])[])'
E                 
E                 Full diff:
E                   (
E                 -     '(uint256,uint256[],(int256,bool[2],address[])[])',
E                 +     'tuple',
E                   )

How was it fixed?

Using the existing collapse_if_tuple function on the input, instead of just taking the raw type field from the dictionary.

Todo:

• Clean up commit history
• (not needed) Add or update documentation related to these changes
• Add entry to the release notes

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