fix flake8 error in master

[dlstadther]

Description

Fixes flake8 error on master branch

Motivation and Context

Failing master build

Have you tested this? If so, how?

Ran flake8 tests locally.

Before change:

flake8 run-test: commands[0] | flake8 --exclude=doc,.tox
./test/worker_test.py:1362:118: E231 missing whitespace after ','
ERROR: InvocationError for command /Users/dlstadther/code/luigi/.tox/flake8/bin/flake8 --exclude=doc,.tox (exited with code 1)
ERROR:   flake8: commands failed

After change:

flake8 run-test: commands[1] | flake8 --max-line-length=100 --ignore=E265 doc
  flake8: commands succeeded
  congratulations :)

[dlstadther]

Thanks for the merge, @rafaelraposospot !