Allow more decimal places for tax rates (#6092)

* Allow more decimal places for tax rates

* Fix normalization

* More tax rate handling

* Add uncommitted tests

* Rebase migration

* Update src/pretix/base/migrations/0304_tax_rate_decimals.py

Co-authored-by: robbi5 <richt@rami.io>

---------

Co-authored-by: robbi5 <richt@rami.io>

src/pretix/helpers/models.py

@@ -20,9 +20,11 @@
 # <https://www.gnu.org/licenses/>.
 #
 import copy
+from decimal import Decimal
 
 from django.core.files import File
 from django.db import models
+from django.db.models.fields import DecimalField
 
 
 class Thumbnail(models.Model):
@@ -54,3 +56,33 @@ def flatten_choices(choices):
             yield from label_or_nested
         else:
             yield value_or_group, label_or_nested
+
+
+def _normalize_decimal(d: Decimal) -> Decimal:
+    """
+    Strips trailing zeros, e.g.
+
+    20.000 → 20
+    20.010 → 20.01
+    20.100 → 20.1
+
+    But unlike of Decimal.normalize(), 20.000 will not become 2e+1. Very small decimals might still be represented
+    in scientific notation when printed.
+    """
+    normalized = d.normalize()
+    sign, digit, exponent = normalized.as_tuple()
+    if exponent > 0:
+        return normalized.quantize(1)
+    return normalized
+
+
+class NormalizedDecimalField(DecimalField):
+    """
+    Variant of DecimalField that never outputs the trailing zeros, so we always have normalized decimals internally.
+    Use this only for fields where the trailing zeros are pointless (e.g. percentages), not for monetary amounts.
+    """
+
+    def from_db_value(self, value, expression, connection):
+        if value is not None:
+            value = _normalize_decimal(value)
+        return value