fix(?) requirements

requirements/base.in

@@ -29,3 +29,4 @@ fastjsonschema
 tweepy
 requests-cache
 wand
+tatsu<=5.8.3

requirements/base.txt

@@ -247,8 +247,10 @@ sqlparse==0.4.4
     # via django
 stack-data==0.6.3
     # via ipython
-tatsu==5.11.3
-    # via ics
+tatsu==5.8.3
+    # via
+    #   -r base.in
+    #   ics
 telepath==0.3.1
     # via wagtail
 tinycss2==1.2.1