Potential multiprocessing issue on Windows with 64+ cores

[amyreese]

Seeing the following exceptions on a Windows machine with 64 cores when running trailrunner via ufmt:

ValueError: need at most 63 handles, got a sequence of length 63

Setting max_workers=X where X < 60 seems to be a solution, but should verify that setting max_workers=60 doesn't mean we spawn 60 child processes even if there are fewer jobs to complete.