Avoid user assigning unimplemented fields on containers.

[JulienPalard]

Before:

doc.trade.settlement.payment_means.type_code = "30"

was allowed but had no effect as payemnt_means is a container.

Now it gives:

Traceback (most recent call last):
  File "/home/mdk/src/python-drafthorse/test.py", line 54, in <module>
    doc.trade.settlement.payment_means.type_code = "30"  # Virement
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Container' object has no attribute 'type_code' and no __dict__ for setting new attributes

It allowed to find issues in the tests, but they are fixed in #100 to avoid conflicts between those two PRs:

There as a:

doc.trade.settlement.payment_means.type_code = "ZZZ"

Instead of:

doc.trade.settlement.payment_means.add(PaymentMeans(type_code="ZZZ"))

And there was a:

doc.trade.settlement.advance_payment.received_date = datetime.now(timezone.utc)

Instead of:

advance = AdvancePayment(
    received_date=datetime.now(timezone.utc), paid_amount=Decimal(42)
)
advance.included_trade_tax.add(
    IncludedTradeTax(
        calculated_amount=Decimal(0),
        type_code="VAT",
        category_code="E",
        rate_applicable_percent=Decimal(0),
    )
)
doc.trade.settlement.advance_payment.add(advance)

[hartwork]

@raphaelm git bisect shows that merging this broke CI.

[hartwork]

Why?!

[JulienPalard]

Why?!

The old README had an invalid line:

doc.trade.settlement.payment_means.type_code = "ZZZ"

This invalid line is detected by this PR, making the CI fail.

But I had another PR to fix the README that has been merged too, with:

doc.trade.settlement.payment_means.add(PaymentMeans(type_code="ZZZ"))

[hartwork]

@JulienPalard thanks for elaborating 👍