Added repr string for Grouper and TimeGrouper
- [ ] closes #xxxx
- [ ] tests added / passed
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This is a new implementation of #17727. This is much simpler than the previous attempts. The outputs are now:

```python
>>> pd.Grouper(key='A')
Grouper(key='A')
>>> pd.Grouper(key='A', freq='2T')
TimeGrouper(key='A', freq=<2 * Minutes>, sort=True, closed='left', label='left', convention='e')
```

Tests still need to be written. I can do them when/if this is ok.