tests: Added xfail to complex abi decoding test for struct

NOTE: This works in reality (see
ethereum/web3.py#1634 (comment))
however our test suite currently can't handle it because of a bug in
Web3.py

tests/parser/globals/test_getters.py

@@ -1,3 +1,6 @@
+import pytest
+
+
 def test_state_accessor(get_contract_with_gas_estimation_for_constants):
     state_accessor = """
 y: HashMap[int128, int128]
@@ -17,6 +20,9 @@ def foo() -> int128:
     assert c.foo() == 5
 
 
+# TODO: Either wait for this, or refactor test suite to use Brownie
+#       (which doesn't suffer from this issue)
+@pytest.mark.xfail(reason="https://github.com/ethereum/web3.py/issues/1634#issuecomment-650797252")
 def test_getter_code(get_contract_with_gas_estimation_for_constants):
     getter_code = """
 struct W: