Flake8 errors

[rly]

flake8==3.8.1 catches a few new flake8 errors that are making CI tests fail:

.\src\hdmf\container.py:342:26: E741 ambiguous variable name 'l'
.\src\hdmf\backends\hdf5\h5tools.py:790:5: C901 'HDF5IO.write_dataset' is too complex (31)
.\src\hdmf\backends\hdf5\h5tools.py:1182:29: E741 ambiguous variable name 'l'
.\src\hdmf\backends\hdf5\h5_utils.py:392:69: E225 missing whitespace around operator
.\src\hdmf\build\builders.py:206:13: E741 ambiguous variable name 'l'
.\src\hdmf\common\table.py:201:5: C901 'DynamicTable.__init__' is too complex (32)
.\src\hdmf\common\table.py:558:23: F507 '...' % ... has 1 placeholder(s) but 2 substitution(s)
.\src\hdmf\validate\validator.py:402:5: C901 'GroupValidator.validate' is too complex (21)
.\tests\coloredtestrunner.py:94:26: E741 ambiguous variable name 'l'
.\tests\unit\test_io_hdf5.py:24:17: E722 do not use bare 'except'
.\tests\unit\spec_tests\test_spec_write.py:54:1: E122 continuation line missing indentation or outdented
.\tests\unit\spec_tests\test_spec_write.py:71:1: E122 continuation line missing indentation or outdented
.\tests\unit\spec_tests\test_spec_write.py:200:1: E122 continuation line missing indentation or outdented