PERF: ExpandingGroupby
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

```
       before           after         ratio
     [89cc5bf9]       [6af880e7]
     <master>         <feature/groupby_expanding_indexer>
-      45.5±0.6ms      3.78±0.04ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'max')
-      46.5±0.5ms      3.86±0.06ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'mean')
-      45.4±0.4ms      3.74±0.05ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'min')
-      45.5±0.4ms      3.73±0.06ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'min')
-      46.6±0.3ms      3.80±0.01ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'median')
-        46.7±1ms      3.77±0.05ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'max')
-      46.3±0.6ms      3.73±0.02ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'sum')
-      46.8±0.3ms       3.76±0.1ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'sum')
-      46.7±0.5ms      3.75±0.02ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'mean')
-      47.2±0.3ms      3.78±0.04ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'median')
-      51.2±0.7ms       3.89±0.1ms     0.08  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'std')
-      51.3±0.3ms      3.78±0.02ms     0.07  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'std')
-      62.6±0.6ms      3.98±0.02ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'int', 'count')
-      62.2±0.7ms      3.91±0.03ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'int', 'count')
-      64.3±0.7ms      3.98±0.05ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'sum')
-      63.5±0.7ms      3.92±0.01ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'max')
-      63.6±0.6ms      3.91±0.03ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'min')
-      63.2±0.7ms      3.86±0.02ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'min')
-        64.2±1ms      3.89±0.04ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'max')
-      64.8±0.8ms      3.91±0.06ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'sum')
-      65.2±0.6ms      3.92±0.05ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'mean')
-      65.2±0.9ms      3.90±0.05ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'median')
-      65.5±0.5ms      3.91±0.02ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'median')
-      65.3±0.2ms      3.86±0.01ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'mean')
-      69.4±0.8ms      4.01±0.07ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'std')
-      69.8±0.6ms      3.93±0.01ms     0.06  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'std')
-      80.1±0.2ms       4.24±0.1ms     0.05  rolling.ExpandingMethods.time_expanding_groupby('DataFrame', 'float', 'count')
-      80.5±0.9ms      4.14±0.02ms     0.05  rolling.ExpandingMethods.time_expanding_groupby('Series', 'float', 'count')

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
```

> do we expose the GroupIndexer in pandas.api.indexers?

No. I don't think GroupByIndexer should be public though great thanks