Calendar Popup: Fix prev/next year for shorter February

Patch by Ludwig Reiter <ludwig.reiter@intevation.de>

diff -r 050e9fd21762 roundup/cgi/templating.py
--- a/roundup/cgi/templating.py	Fri Nov 15 10:41:15 2019 +0100
+++ b/roundup/cgi/templating.py	Fri Nov 15 10:43:26 2019 +0100
@@ -3140,6 +3140,20 @@
             days = calendar.monthrange(year, month)[1]
             return date.Interval("+%dd" % days)
 
+        def prev_year_interval(display):
+            # jump to the correct month in the prev year
+            # try to handle the special 29/02 case.
+            if display.month == 2 and display.day == 29:
+                return date.Interval("-366d") 
+            return date.Interval("-1y")
+
+        def next_year_interval(display):
+            # jump to the correct month in the next year
+            # try to handle the special 29/02 case.
+            if display.month == 2 and display.day == 29:
+                return date.Interval("+364d")
+            return date.Interval("+1y")
+
         # for navigation
         try:
             date_prev_month = display + prev_month_interval(display)
@@ -3150,11 +3164,11 @@
         except ValueError:
             date_next_month = None
         try:
-            date_prev_year = display + date.Interval("-1y")
+            date_prev_year = display + prev_year_interval(display)
         except ValueError:
             date_prev_year = None
         try:
-            date_next_year = display + date.Interval("+1y")
+            date_next_year = display + next_year_interval(display)
         except ValueError:
             date_next_year = None