Fix num_days in PandasAutoDateLocator
Prior to this commit, one year is worth 12 days, not 12 * 31 days.
Could you add a release note to `whatsnew/v0.19.2.txt`? Tests will be hard I think, but looking at the diff, your logic looks correct. is there an open issue for this? ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14716?src=pr) is 85.21% (diff: 100%)
> Merging [#14716](https://codecov.io/gh/pandas-dev/pandas/pull/14716?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #14716   diff @@
==========================================
  Files           143        143          
  Lines         50796      50796          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43284      43284          
  Misses         7512       7512          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [fdb70a9...7f78fda](https://codecov.io/gh/pandas-dev/pandas/compare/fdb70a96716c07a0940312cabffb61aea5d6d997...7f78fdaf4b1fd780a773e341f90129178bf8a54f?src=pr) can you add a whatsnew note (0.20.0) and is there a test / example that shows the current behavior (is incorrect)? can you rebase @jorisvandenbossche ? See discussion in https://github.com/pandas-dev/pandas/issues/15753. Merging this as it was obviously a typo in the code, but one that will not affect any behaviour.