Taxe Rate: Use same rounding strategy as Stripe for tax amounts

[feliixx]

When computing taxes, Stripe uses the 'commercial round' strategy, meaning that
0.5 will be rounded to 1 (when rounding to an integer) as stated here:
https://docs.stripe.com/billing/taxes/tax-rates#rounding

Localstripe behaves differently, because to achieve rounding it casts the
resulting float to an int with the line:

'amount': int(amount * self.percentage / 100.0)

But casting to an int doesn't use the same rounding strategy, indeed the
Python documentation says:

For floating-point numbers, this truncates towards zero.

So this effectively rounds 0.5 to 0, which is different from Stripe behavior.

This actually cause some computation differences for some combinations of tax
rate and amount, so let's update _tax_amount rounding strategy to use the same
one as Stripe.

WARNING: This is a breaking change as it will change the computed total of an
invoice in some scenarios.

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Side note: I made sure that this change doesn't break our internal 11* test suit

[adrienverge]

This makes much sense, thanks 👍

As you point out, this is a breaking change. If anyone has a good reason to be opposed to this, please tell it quickly 🙏

[feliixx]

Thanks for the review ! Nobody raised any concern about this so let's merge