Add compat function for date.fromisocalendar for Python 3.6-3.7

src/pretix/helpers/compat.py

@@ -0,0 +1,9 @@
+import datetime
+import sys
+
+
+def date_fromisocalendar(isoyear, isoweek, isoday):
+    if sys.version_info < (3, 8):
+        return datetime.datetime.strptime(f'{isoyear}-W{isoweek}-{isoday}', "%G-W%V-%u")
+    else:
+        return datetime.datetime.fromisocalendar(isoyear, isoweek, isoday)