### Steps to reproduce the issue:

1. Activate Mexican Localization
2. With a Mexican Company, create an Invoice and add a Product with a valid UNSPSC code
3. On the Invoice Line, set Price to 1030.17 and add a 3% Discount
4. Confirm then Send & Print (with CFDI checked)
5. In the generated XML, the following values are off:
    - In node "Comprobante, SubTotal="1030.16" Descuento="30.90"
    - In node "Concepto", ValorUnitario="1030.16" Importe="1030.16" Descuento="30.90"
    - Expected are 1030.17 and 30.91

### Explanation:

During the calculation of the values for the CFDI, `gross_price_subtotal_before_discount` is calculated using `price_subtotal` in every case where `discount` is not 100%. This is due to other values affecting `price_subtotal` (e.g.: taxes with `price_include=True`), in which case calculating a value related to `price_subtotal` with `price_unit` would give the wrong result.
This calculation method causes rounding issues such as above because `price_subtotal` is rounded before calculating `gross_price_subtotal_before_discount`.

### Fix reasoning:

We need to account for any value that could affect `price_subtotal` and make it different from `price_unit * quantity` except for `discount`, which will be included when comparing both values.
The case `discount == 100.0` is covered by this new condition as well, since `discount_factor` would equal to 0 and `price_subtotal` too.

opw-4183556