Tarjan's Algorithm Python Implementation
Multi-statement lines

This:

stack = []; result = []
lowlinks = {}; index = {}
# ...
if successor == node: break


is generally discouraged; just use two lines.

snake_case

This:

lowlinks

is usually spelled out, i.e. low_links.

Bare except

    try:
        successors = graph[node]
    except:
        successors = []


has an except statement that's too broad. If you expect a KeyError from this, then except KeyError.