CLN: ASV string
Cleaned up the `strings.py` benchmarks, `pandas_vb_common.py` of all the unnecessary imports, and linting all the `.py` files in `asv_bench` now.

```
asv dev -b ^strings
· Discovering benchmarks
· Running 25 total benchmarks (1 commits * 1 environments * 25 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  4.00%] ··· Running strings.Contains.time_contains                         ok
[  4.00%] ···· 
               ======= ========
                regex          
               ------- --------
                 True   107ms  
                False   38.4ms 
               ======= ========

[  8.00%] ··· Running strings.Dummies.time_get_dummies                    7.80s
[ 12.00%] ··· Running strings.Encode.time_encode_decode                   697μs
[ 16.00%] ··· Running strings.Methods.time_cat                           24.1ms
[ 20.00%] ··· Running strings.Methods.time_center                        84.2ms
[ 24.00%] ··· Running strings.Methods.time_count                          119ms
[ 28.00%] ··· Running strings.Methods.time_endswith                      68.0ms
[ 32.00%] ··· Running strings.Methods.time_extract                        477ms
[ 32.00%] ····· /home/matt/Projects/pandas-mroeschke/asv_bench/benchmarks/strings.py:26: FutureWarning: currently extract(expand=None) means expand=False (return Index/Series/DataFrame) but in a future version of pandas this will be changed to expand=True (return DataFrame)
                self.s.str.extract('(\\w*)A(\\w*)')

[ 36.00%] ··· Running strings.Methods.time_findall                        173ms
[ 40.00%] ··· Running strings.Methods.time_get                           59.9ms
[ 44.00%] ··· Running strings.Methods.time_len                           41.1ms
[ 48.00%] ··· Running strings.Methods.time_lower                         56.3ms
[ 52.00%] ··· Running strings.Methods.time_lstrip                        56.3ms
[ 56.00%] ··· Running strings.Methods.time_match                          143ms
[ 60.00%] ··· Running strings.Methods.time_pad                           85.6ms
[ 64.00%] ··· Running strings.Methods.time_replace                       73.2ms
[ 68.00%] ··· Running strings.Methods.time_rstrip                        59.1ms
[ 72.00%] ··· Running strings.Methods.time_slice                         51.9ms
[ 76.00%] ··· Running strings.Methods.time_startswith                    67.2ms
[ 80.00%] ··· Running strings.Methods.time_strip                         56.0ms
[ 84.00%] ··· Running strings.Methods.time_title                         59.3ms
[ 88.00%] ··· Running strings.Methods.time_upper                         55.8ms
[ 92.00%] ··· Running strings.Repeat.time_repeat                             ok
[ 92.00%] ···· 
               ========= ========
                repeats          
               --------- --------
                  int     77.7ms 
                 array    70.2ms 
               ========= ========

[ 96.00%] ··· Running strings.Slice.time_vector_slice                     271ms
[100.00%] ··· Running strings.Split.time_split                               ok
[100.00%] ···· 
               ======== =======
                expand         
               -------- -------
                 True    773ms 
                False    352ms 
               ======== =======
```
