Python: Change all remaining occurrences of StrConst

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

python/ql/test/3/extractor-tests/fstrings3.6/Formatted.ql

@@ -8,7 +8,7 @@ where
     not exists(val.getConversion()) and typeconv = " "
   ) and
   (
-    format = val.getFormatSpec().getValue(0).(StrConst).getText()
+    format = val.getFormatSpec().getValue(0).(StringLiteral).getText()
     or
     not exists(val.getFormatSpec()) and format = ""
   )

python/ql/test/3/extractor-tests/fstrings3.6/Successors.ql

@@ -1,9 +1,9 @@
 import python
 
 string repr(AstNode a) {
-  not a instanceof StrConst and result = a.toString()
+  not a instanceof StringLiteral and result = a.toString()
   or
-  result = "\"" + a.(StrConst).getText() + "\""
+  result = "\"" + a.(StringLiteral).getText() + "\""
 }
 
 from ControlFlowNode p, ControlFlowNode s, BasicBlock b, int n

python/ql/test/3/extractor-tests/fstrings3.8/Successors.ql

@@ -1,9 +1,9 @@
 import python
 
 string repr(AstNode a) {
-  not a instanceof StrConst and result = a.toString()
+  not a instanceof StringLiteral and result = a.toString()
   or
-  result = "\"" + a.(StrConst).getText() + "\""
+  result = "\"" + a.(StringLiteral).getText() + "\""
 }
 
 from ControlFlowNode p, ControlFlowNode s, BasicBlock b, int n

python/ql/test/3/extractor-tests/multibyte/Test.ql

@@ -1,4 +1,4 @@
 import python
 
-from StrConst s
+from StringLiteral s
 select s, s.getText()

python/ql/test/3/library-tests/locations/general/Prefix.ql

@@ -1,4 +1,4 @@
 import python
 
-from StrConst s
+from StringLiteral s
 select s.getLocation().getStartLine(), s.getText(), s.getPrefix()