flake8: fix ambiguous var names

[pmrowla]

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Fixes errors from latest flake8 master (pycodestyle >= 2.6.0a1, < 2.7.0)

[INFO] This may take a few minutes...
black....................................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1
tests/func/test_repro.py:1558:39: E741 ambiguous variable name 'l'
dvc/dagascii.py:98:41: E741 ambiguous variable name 'l'
beautysh.................................................................Passed
DVC pre-commit...........................................................Passed
- hook id: dvc-pre-commit
- duration: 16.59s