[mypy] Type annotations for `graphs/finding_bridges.py` and `graphs/random_graph_generator.py` (#5795)

* [mypy] Annotate `graphs/finding_bridges.py`

* Remove from excluded in `mypy.ini`

* Add doctest.testmod()

* psf/black formatting

* Annotations for `graphs/random_graph_generator.py`

* Remove from excluded in `mypy.ini`

* Resolve merge conflict

* Resolve merge conflict

* Update mypy.ini

* Update mypy.ini

* Remove from excluded