Cast timestamp epoch into FLOAT to avoid rounding

On PostgreSQL-12, the epoch FLOAT is now 6 digits but rounded to 5 digits when
cast into NUMERIC.

issue8779
review264501002

trytond/model/modelsql.py

@@ -539,7 +539,7 @@ class ModelSQL(ModelStorage):
                         '%s,%s' % (cls.__name__, id_))
                 except KeyError:
                     continue
-                sql_type = fields.Numeric('timestamp').sql_type().base
+                sql_type = fields.Float('timestamp').sql_type().base
                 where.append((table.id == id_)
                     & (Extract('EPOCH',
                             Coalesce(table.write_date, table.create_date)

trytond/tests/test_modelsql.py

@@ -293,6 +293,7 @@ class ModelSQLTestCase(unittest.TestCase):
             # timestamp precision of sqlite is the second
             time.sleep(1)
 
+        transaction.timestamp[str(record)] = timestamp
         ModelsqlTimestamp.write([record], {})
         transaction.commit()