STY: Boolean values for bint variables
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

---

```asv``` benchmarks: to show that performance is not changed with the stylistic changes)

```
All benchmarks:

       before           after         ratio
     [28e0f18a]       [6f1fd78d]
     <master>         <STY-bint-boolean-not-int-join>
         21.4±1ms       19.8±0.4ms     0.92  join_merge.Join.time_join_dataframe_index_multi(False)
       24.9±0.3ms       23.6±0.3ms     0.95  join_merge.Join.time_join_dataframe_index_multi(True)
       14.5±0.3ms       14.0±0.3ms     0.97  join_merge.Join.time_join_dataframe_index_shuffle_key_bigger_sort(False)
       17.6±0.6ms       16.7±0.5ms     0.95  join_merge.Join.time_join_dataframe_index_shuffle_key_bigger_sort(True)
       14.4±0.3ms       13.8±0.2ms     0.95  join_merge.Join.time_join_dataframe_index_single_key_bigger(False)
       17.8±0.7ms       16.4±0.5ms     0.92  join_merge.Join.time_join_dataframe_index_single_key_bigger(True)
       13.5±0.3ms       13.1±0.3ms     0.97  join_merge.Join.time_join_dataframe_index_single_key_small(False)
       14.5±0.1ms       13.9±0.5ms     0.96  join_merge.Join.time_join_dataframe_index_single_key_small(True)
       2.86±0.08s       2.94±0.05s     1.03  join_merge.JoinIndex.time_left_outer_join_index
         433±70μs         428±50μs     0.99  join_merge.JoinNonUnique.time_join_non_unique_equal
```