rethink typing for FinStatementsBase considering invariant types

[nickderobertis]

Was trying to set generic base types in the base class FinStatementsBase
and then in the subclasses, set them to the specific types. But this seems to not
work correctly with mutable collections of the types.

Currently I have set type ignore for all the subclass typing

See python/mypy#2984 (comment) for more details

---

This issue has been automatically created by todo-actions based on a TODO comment found in finstmt/findata/statementsbase.py:20. It will automatically be closed when the TODO comment is removed from the default branch (master).