Advent of Code day 7 part 2

from collections import defaultdict
from heapq import heapify, heappop, heappush
import re
from string import ascii_uppercase

def day7_digraph():
    out, in_ = defaultdict(set), defaultdict(set)
    with open('7.txt') as f:
        for line in f:
            m = re.match(r'Step (.) must be finished before step (.)', line)
            src, dest = m.groups()
            out[src].add(dest)
            in_[dest].add(src)
    return out, in_

def day7_part2(workers=5, duration=60):
    out, in_ = day7_digraph()
    available_tasks = [t for t in out if not in_[t]]
    heapify(available_tasks)
    busy_workers = [(0, None)]
    while busy_workers:
        time, task = heappop(busy_workers)
        for t in out[task]:
            deps = in_[t]
            deps.remove(task)
            if not deps:
                heappush(available_tasks, t)
        while available_tasks and len(busy_workers) < workers:
            task = heappop(available_tasks)
            finish = time + duration + ascii_uppercase.index(task) + 1
            heappush(busy_workers, (finish, task))
    return time