kfet -- Fix available checkouts in K-Psul

The checkout validity is checked using the current datetime (when
requesting the kpsul page).

kfet/forms.py

@@ -296,17 +296,17 @@ class KPsulAccountForm(forms.ModelForm):
 
 class KPsulCheckoutForm(forms.Form):
     checkout = forms.ModelChoiceField(
-        queryset=(
-            Checkout.objects
-            .filter(
-                is_protected=False,
-                valid_from__lte=timezone.now(),
-                valid_to__gte=timezone.now(),
-            )
-        ),
+        queryset=None,
         widget=forms.Select(attrs={'id': 'id_checkout_select'}),
     )
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        # Create the queryset on form instanciation to use the current time.
+        self.fields['checkout'].queryset = (
+            Checkout.objects.is_valid().filter(is_protected=False))
+
 
 class KPsulOperationForm(forms.ModelForm):
     article = forms.ModelChoiceField(