[FIX] account: iteration different under ordereddict mutation

Did you know that if you iterate on an OrderedDict in Python 2 and add
to the dict during iteration, the new items will be iterated? No? Me
neither.

And did you know that that does not work in Python 3 likely because
OrderedDict's implementation details changed? Why the fuck would
anyone do that and thus why would we care you ask?

WELL WE FUCKING DO IT TURNS OUT and that breaks
test_partial_reconcile_currencies on Python 3...

Fix it, try to make it more obvious that we're doing weird crap
instead of abusing OrderedDict in ways it was never meant to be used.

addons/account/models/account_move.py

@@ -1722,17 +1722,20 @@ class AccountPartialReconcile(models.Model):
             return self
         #check if the reconcilation is full
         #first, gather all journal items involved in the reconciliation just created
-        partial_rec_set = OrderedDict.fromkeys([x for x in self])
         aml_set = aml_to_balance = self.env['account.move.line']
         total_debit = 0
         total_credit = 0
         total_amount_currency = 0
         #make sure that all partial reconciliations share the same secondary currency otherwise it's not
         #possible to compute the exchange difference entry and it has to be done manually.
-        currency = list(partial_rec_set)[0].currency_id
+        currency = self[0].currency_id
         maxdate = '0000-00-00'
 
-        for partial_rec in partial_rec_set:
+        seen = set()
+        todo = set(self)
+        while todo:
+            partial_rec = todo.pop()
+            seen.add(partial_rec)
             if partial_rec.currency_id != currency:
                 #no exchange rate entry will be created
                 currency = None
@@ -1751,13 +1754,15 @@ class AccountPartialReconcile(models.Model):
                         #currency is recorded on the partial rec and in order to check if the reconciliation is total, we need to convert the
                         #aml.balance in that foreign currency
                         total_amount_currency += aml.company_id.currency_id.with_context(date=aml.date).compute(aml.balance, partial_rec.currency_id)
+
                 for x in aml.matched_debit_ids | aml.matched_credit_ids:
-                    partial_rec_set[x] = None
-        partial_rec_ids = [x.id for x in partial_rec_set]
+                    if x not in seen:
+                        todo.add(x)
+
+        partial_rec_ids = [x.id for x in seen]
         aml_ids = aml_set.ids
         #then, if the total debit and credit are equal, or the total amount in currency is 0, the reconciliation is full
         digits_rounding_precision = aml_set[0].company_id.currency_id.rounding
-
         if (currency and float_is_zero(total_amount_currency, precision_rounding=currency.rounding)) or float_compare(total_debit, total_credit, precision_rounding=digits_rounding_precision) == 0:
             exchange_move_id = False
             exchange_partial_rec_id = False