STYLE: Fix linting of benchmarks
We are currently not linting (validating PEP-8) of the benchmarks in `asv_bench/benchmarks`. But it'd be good to start doing it as with the rest of the code.

Before starting the automatic validation, we need to fix all the style problems. Those can be obtained by executing `flake8 --config=none asv_bench/benchmarks --ignore=F811,E731`

This is currently returning:
```
asv_bench/benchmarks/algorithms.py:12:5: E722 do not use bare except'
asv_bench/benchmarks/timeseries.py:1:1: F401 'warnings' imported but unused
asv_bench/benchmarks/stat_ops.py:21:9: E722 do not use bare except'
asv_bench/benchmarks/stat_ops.py:59:9: E722 do not use bare except'
asv_bench/benchmarks/pandas_vb_common.py:5:1: F401 'pandas.Panel' imported but unused
asv_bench/benchmarks/pandas_vb_common.py:12:5: E722 do not use bare except'
asv_bench/benchmarks/pandas_vb_common.py:37:9: E722 do not use bare except'
asv_bench/benchmarks/join_merge.py:32:9: E722 do not use bare except'
asv_bench/benchmarks/io/csv.py:2:1: F401 'timeit' imported but unused
asv_bench/benchmarks/io/csv.py:8:1: F401 'pandas.compat.PY2' imported but unused
asv_bench/benchmarks/io/csv.py:184:80: E501 line too long (87 > 79 characters)
```

All them should be fixed. Most of them should be immediate to fix, but for `E722 do not use bare except` make sure the specific exception is captured, and don't simply use `except Exception:`.