Python: Add Str class

This makes the AST viewer (which annotates string constant nodes as
`Str`) a bit more consistent.

python/ql/lib/semmle/python/Exprs.qll

@@ -616,8 +616,11 @@ private string non_byte_prefix() {
   not result.charAt(_) in ["b", "B"]
 }
 
+/** A string constant. This is a placeholder class -- use `StrConst` instead. */
+class Str extends Str_ { }
+
 /** A string constant. */
-class StrConst extends Str_, ImmutableLiteral {
+class StrConst extends Str, ImmutableLiteral {
   /* syntax: "hello" */
   predicate isUnicode() {
     this.getPrefix() = unicode_prefix()