[FIX] payment_payulatam: use "Round half to even" method in signature

Steps:
- Install l10n_ar, website_sale
- Go to Website / Configuration / eCommerce / Payment Acquirers
- Install PayUlatam
- Edit PayUlatam:
  - Argentinian credentials: http://developers.payulatam.com/en/web_checkout/sandbox.html
- Go to Website / Configuration / Settings
- Check Multi-Currencies
- Save
- Click Currencies
- Edit all currencies:
  - Rates:
    - Edit or create the current rate:
      - Rate: 1.000000
- Go to Website / Products / Pricelists
- Create an ARS pricelist and make it selectable
- Go to Website / Products / Products
- Create a product (1):
  - Sales Price: 15.75
  - No taxes
- Click "Published On Website"
- Select your new ARS pricelist
- Add (1) to Cart
- Process Checkout
- Select PayUlatam
- Pay Now
- Use this info to process the checkout: http://developers.payulatam.com/en/web_checkout/sandbox.html
- Pagar (Pay)
- Regresar al sitio de la tienda (Back to the store website)

Bug:
500: PayUlatam: invalid sign

Explanation:
From PayU's documentation:
> 2. Response page
> Signature validation
> In order to get the new value new_value you always should approximate
> the TX_VALUE to a decimal, using the rounding method "Round half to
> even"
http://developers.payulatam.com/en/web_checkout/integration.html

opw:2457240

closes odoo/odoo#68816

X-original-commit: 5f703cd92293daa1049cbeced9515b251d69bd02
Signed-off-by: backspac <backspac@users.noreply.github.com>

addons/payment_payulatam/models/payment_acquirer.py

@@ -50,9 +50,11 @@ class PaymentAcquirer(models.Model):
                 self.payulatam_api_key,
                 self.payulatam_merchant_id,
                 values['referenceCode'],
-                # Observation from the sandbox: PayU Latam is using a rounded value to generate
-                # their signature despite saying otherwise in the doc:
                 # http://developers.payulatam.com/en/web_checkout/integration.html
+                # Section: 2. Response page > Signature validation
+                # PayU Latam use the "Round half to even" rounding method
+                # to generate their signature. This happens to be Python 3's
+                # default rounding method.
                 float_repr(float(values.get('TX_VALUE')), 1),
                 values['currency'],
                 values.get('transactionState'),