Python: Add tracking steps for class level attributes

[naneer]

Adds support for type tracking for class level attributes and default instance variables. For example, to identify missing SQLExecution sinks for the SQLInjection DataFlow Rule. Also supports values inherited from base classes.

These SQLExecution sinks will be found:

from django.views import View
from django.db import connection

class ClassBasedView(View):
  _connection = connection
  def get(self, request):
     ...
     with self._connection.cursor() as cursor:
       cursor.execute("SELECT ... FROM ... WHERE")
     ...
  ...

class ClassBasedViewInit(View):
  def __init__(self, conn = connection):
    self._connection = conn
  ...
  def get(self, request):
    ...
    with self._connection.cursor() as cursor:
       cursor.execute("SELECT ... FROM ... WHERE")
    ...
  ...

// Inherited from base classes

class SubClassBasedView(ClassBasedView):
  def get(self, request):
    ...
    with self._connection.cursor() as cursor:
       cursor.execute("SELECT ... FROM ... WHERE")
    ...
  ...

class SubClassBasedViewInit(ClassBasedViewInit):
  def get(self, request):
    ...
    with self._connection.cursor() as cursor:
       cursor.execute("SELECT ... FROM ... WHERE")
    ...
  ...

[RasmusWL]

Thanks for your work on this 💪 I have actually been working on this as well, so I'll have a PR coming up shortly on this. Since most of your code uses points-to analysis that we are moving away from, we wouldn't be able to accept the actual implementation, but I really do appreciate the effort 👍

[naneer]

Ok, no problem. Looking forward to it!