i18n.py: declare plural forms for the Turkish language

I'm not quite sure, but it seems to me that Qt Linguist treats the
Turkish language as having one "universal form". We need plural forms
for Turkish since FGData commit c0bb8d8a8d64e1 in branch
'release/2020.3' added a 'tr' translation.

python3-flightgear/flightgear/meta/i18n.py

@@ -98,6 +98,7 @@ PLURAL_FORMS = {
     "pt": ["singular", "plural"],
     "ru": ["singular", "plural"],
     "sk": ["singular", "paucal", "plural"],
+    "tr": ["universal"],
     "zh": ["universal"]         # universal form
 }