[Python] tsg-parser fails to extract generic base class relations

首先重現 class Bar(list[int]): pass 的 v2.26.2 tsg-python 輸出，然後追蹤 parser 在建構類別基底時如何處理 Subscript 節點。完成的標準是：泛型基底產生一條 bases 邊，且 CodeQL 能辨識 Bar -> list[int] 關係，同時現有的非泛型情況維持不變。

Working with typed base classes in Python (see #22291) I realised that the toolchain seems to drop generic base classes (e.g. list[int] in class Bar(list[int]):) at some point. That is, in the example code

class Foo(list): pass

class Bar(list[int]): pass

CodeQL knows the Foo -> list relation but not the Bar -> list[int] relation.

As far as I could trace it, the relation is already lost in the tsg-python parser. Using the v2.26.2 toolchain, the source code class Bar(list): pass produces the following tsg-python output (assignment nodes removed for brevity):

node 4
  _kind: "Name"
  _location: [0, 10, 0, 14]
  ctx: "load"
  variable: "list"
node 5
  _kind: "ClassExpr"
  _location: [0, 0, 0, 21]
  _location_end: [0, 16]
  inner_scope: [graph node 6]
  name: "Bar"
edge 5 -> 4
  bases: 0
node 6
  _kind: "Class"
  _location: [0, 0, 0, 21]
  _location_end: [0, 16]
  name: "Bar"
edge 6 -> 1
  body: 0

Now if the code is extended for a generic subscription as class Bar(list[int]): pass produces the following tsg-python output (assignment nodes removed for brevity):

node 4
  _kind: "Name"
  _location: [0, 10, 0, 14]
  ctx: "load"
  variable: "list"
node 5
  _kind: "Name"
  _location: [0, 15, 0, 18]
  ctx: "load"
  variable: "int"
node 6
  _kind: "Subscript"
  _location: [0, 10, 0, 19]
  ctx: "load"
  index: [graph node 5]
  value: [graph node 4]
node 7
  _kind: "ClassExpr"
  _location: [0, 0, 0, 26]
  _location_end: [0, 21]
  inner_scope: [graph node 8]
  name: "Bar"
node 8
  _kind: "Class"
  _location: [0, 0, 0, 26]
  _location_end: [0, 21]
  name: "Bar"
edge 8 -> 1
  body: 0

Notice that the type subscription is parsed (nodes 4, 5, 6) but the edge n -> m node for the bases: 0 relation is absent.