Python: Refactor ExternalFileObject to use field

Instead of string matching. This brings it in line with what CollectionKind,
SequenceKind, and DictKind does.

python/ql/src/semmle/python/security/strings/External.qll

@@ -183,10 +183,12 @@ private predicate urlparse(ControlFlowNode fromnode, CallNode tonode) {
 
 /** A kind of "taint", representing an open file-like object from an external source. */
 class ExternalFileObject extends TaintKind {
-    ExternalFileObject() { this = "file[" + any(ExternalStringKind key) + "]" }
+    ExternalStringKind valueKind;
+
+    ExternalFileObject() { this = "file[" + valueKind + "]" }
 
     /** Gets the taint kind for the contents of this file */
-    TaintKind getValue() { this = "file[" + result + "]" }
+    TaintKind getValue() { result = valueKind }
 
     override TaintKind getTaintOfMethodResult(string name) {
         name = "read" and result = this.getValue()