Python: Change all remaining occurrences of StrConst

Done using
```
git grep StrConst | xargs sed -i 's/StrConst/StringLiteral/g'
```

python/ql/examples/snippets/raw_string.ql

@@ -8,6 +8,6 @@
 
 import python
 
-from StrConst s
+from StringLiteral s
 where s.getPrefix().matches("%r%")
 select s

python/ql/examples/snippets/singlequotestring.ql

@@ -9,6 +9,6 @@
 
 import python
 
-from StrConst s
+from StringLiteral s
 where s.getPrefix().charAt(_) = "'"
 select s