pin flake8

[MarcoGorelli]

cc @tgross35 have you found a version for which all tests pass? I've tried 3.8.0 but get quite a few failures:

FAILED tests/test_precisely.py::test_checker[11] - assert ('test_precis...'tokens': 55}) == ('test_precis...'tok...
FAILED tests/test_precisely.py::test_summary[1] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[10] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[11] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[12] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[13] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[2] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[3] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[4] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[5] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[6] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[7] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[8] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[9] - ValueError: empty separator
FAILED tests/test_precisely.py::test_summary[14] - ValueError: empty separator
FAILED tests/test_source_block.py::test_clean_console_syntax[%timeit a = (1, 2,name)\n-a = (1, 2,name)\n] - Asse...
FAILED tests/test_source_block.py::test_clean_console_syntax[%time a = (1, 2,name)\nb = (3, 4, other)\n-a = (1, 2,name)\nb = (3, 4, other)\n]
FAILED tests/test_source_block.py::test_clean_console_syntax[%time df = pd.read_csv('big.csv')\n-df = pd.read_csv('big.csv')\n]
FAILED tests/test_source_block.py::test_clean_console_syntax[%time df = pd.read_csv("big.csv")\n-df = pd.read_csv("big.csv")\n]
FAILED tests/test_source_block.py::test_clean_console_syntax[%time df = pd.read_csv("big.csv")\n%time df = pd.read_csv('big.csv')\n-df = pd.read_csv("big.csv")\ndf = pd.read_csv('big.csv')\n]
FAILED tests/test_source_block.py::test_ignore_unrecognized_console_syntax[%prun -l 4 f(x)\n] - AssertionError: ...
FAILED tests/test_source_block.py::test_ignore_unrecognized_console_syntax[%%timeit x = range(10000)\nmax(x)\n]
FAILED tests/test_source_block.py::test_roles[   :flake8-{role}:{value} #{comment}\n] - KeyError: '0'