BUG: fix pickling of Custom offsets in 3.6
 xref #14679
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14685?src=pr) is 85.29% (diff: 100%)

> Merging [#14685](https://codecov.io/gh/pandas-dev/pandas/pull/14685?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14685   diff @@
==========================================
  Files           140        140          
  Lines         50706      50708     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43247      43250     +3   
+ Misses         7459       7458     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [748000d...8ad212c](https://codecov.io/gh/pandas-dev/pandas/compare/748000d220c3c7d4daddf5811e10a5acdfce49ea...8ad212c92a86aadffe0fb3b2af394775c221310d?src=pr)
