STYLE: support flake8>3.4.1
flake8 > 3.4.1 has a fair amount more warnings

we need to either fix our code or remove the warnings

example from: https://travis-ci.org/pandas-dev/pandas/jobs/305553688

note that #18418 will pin 3.4.1, so these won't show in the CI.
We will need to revert this commit once things are fixed.

prob ok to turn off E740 (though fixing is not a big deal)
should fix the E722
```
$ ci/lint.sh
inside ci/lint.sh
Linting *.py
pandas/compat/pickle_compat.py:36:13: E722 do not use bare except'
pandas/compat/pickle_compat.py:47:13: E722 do not use bare except'
pandas/compat/pickle_compat.py:171:1: E722 do not use bare except'
pandas/compat/pickle_compat.py:199:5: E722 do not use bare except'
pandas/core/categorical.py:601:9: E722 do not use bare except'
pandas/core/common.py:571:5: E722 do not use bare except'
pandas/core/common.py:589:5: E722 do not use bare except'
pandas/core/common.py:608:5: E722 do not use bare except'
pandas/core/common.py:620:5: E722 do not use bare except'
pandas/core/frame.py:2118:9: E722 do not use bare except'
pandas/core/frame.py:2561:13: E722 do not use bare except'
pandas/core/frame.py:5717:21: E722 do not use bare except'
pandas/core/frame.py:6232:9: E722 do not use bare except'
pandas/core/indexing.py:136:13: E722 do not use bare except'
pandas/core/indexing.py:540:17: E741 ambiguous variable name 'l'
pandas/core/indexing.py:698:21: E741 ambiguous variable name 'l'
```