[FIX] tools/date_utils: Fix get_fiscal_year with leap years

Suppose calling get_fiscal_year(date, day=29, month=2) with date(year=2017, month=2, day=28)

The computation steps are:
date_from = date(year=2017, month=2, day=28) - 1 year = date(year=2016, month=2, day=28)
date_from = date_from + 1 day = date(year=2016, month=2, day=29)

It's incorrect because the value of date_from must be date(year=2016, month=3, day=1)

closes odoo/odoo#29416

addons/account/tests/test_account_fiscal_year.py

@@ -72,8 +72,15 @@ class TestFiscalPosition(AccountingTestCase):
 
         self.check_compute_fiscal_year(
             company,
-            '2016-03-01',
             '2016-02-29',
+            '2015-03-01',
+            '2016-02-29',
+        )
+
+        self.check_compute_fiscal_year(
+            company,
+            '2016-03-01',
+            '2016-03-01',
             '2017-02-28',
         )
 

odoo/tools/date_utils.py

@@ -55,8 +55,19 @@ def get_fiscal_year(date, day=31, month=12):
     '''
     max_day = calendar.monthrange(date.year, month)[1]
     date_to = type(date)(date.year, month, min(day, max_day))
+
+    # Force at 29 February instead of 28 in case of leap year.
+    if date_to.month == 2 and date_to.day == 28 and max_day == 29:
+        date_to = type(date)(date.year, 2, 29)
+
     if date <= date_to:
         date_from = date_to - relativedelta(years=1)
+        max_day = calendar.monthrange(date_from.year, date_from.month)[1]
+
+        # Force at 29 February instead of 28 in case of leap year.
+        if date_from.month == 2 and date_from.day == 28 and max_day == 29:
+            date_from = type(date)(date_from.year, 2, 29)
+
         date_from += relativedelta(days=1)
     else:
         date_from = date_to + relativedelta(days=1)