BUG: fix pickling of Custom offsets in 3.6
BUG: fix pickling of Custom offsets in 3.6

xref #14679

Author: Jeff Reback <jeff@reback.net>

Closes #14685 from jreback/offsets and squashes the following commits:

8ad212c [Jeff Reback] BUG: fix pickling of Custom offsets in 3.6

jreback/pandas,jreback/pandas,gfyoung/pandas,gfyoung/pandas,GuessWhoSamFoo/pandas,TomAugspurger/pandas,Winand/pandas,cython-testbed/pandas,toobaz/pandas,winklerand/pandas,harisbal/pandas,pratapvardhan/pandas,pratapvardhan/pandas,dsm054/pandas,amolkahat/pandas,louispotok/pandas,harisbal/pandas,DGrady/pandas,winklerand/pandas,kdebrab/pandas,louispotok/pandas,Winand/pandas,toobaz/pandas,jreback/pandas,GuessWhoSamFoo/pandas,jmmease/pandas,rs2/pandas,DGrady/pandas,jorisvandenbossche/pandas,cbertinato/pandas,datapythonista/pandas,rs2/pandas,TomAugspurger/pandas,zfrenchee/pandas,kdebrab/pandas,jreback/pandas,louispotok/pandas,jmmease/pandas,cbertinato/pandas,DGrady/pandas,amolkahat/pandas,amolkahat/pandas,dsm054/pandas,jmmease/pandas,pandas-dev/pandas,nmartensen/pandas,datapythonista/pandas,jmmease/pandas,linebp/pandas,MJuddBooth/pandas,nmartensen/pandas,harisbal/pandas,Winand/pandas,linebp/pandas,linebp/pandas,cython-testbed/pandas,GuessWhoSamFoo/pandas,nmartensen/pandas,jorisvandenbossche/pandas,jorisvandenbossche/pandas,cython-testbed/pandas,DGrady/pandas,amolkahat/pandas,harisbal/pandas,winklerand/pandas,gfyoung/pandas,nmartensen/pandas,zfrenchee/pandas,MJuddBooth/pandas,rs2/pandas,GuessWhoSamFoo/pandas,jmmease/pandas,harisbal/pandas,jreback/pandas,zfrenchee/pandas,pandas-dev/pandas,gfyoung/pandas,linebp/pandas,nmartensen/pandas,zfrenchee/pandas,cbertinato/pandas,gfyoung/pandas,pratapvardhan/pandas,winklerand/pandas,cbertinato/pandas,amolkahat/pandas,louispotok/pandas,kdebrab/pandas,louispotok/pandas,pandas-dev/pandas,kdebrab/pandas,rs2/pandas,jmmease/pandas,winklerand/pandas,Winand/pandas,datapythonista/pandas,Winand/pandas,dsm054/pandas,pandas-dev/pandas,zfrenchee/pandas,nmartensen/pandas,toobaz/pandas,TomAugspurger/pandas,pratapvardhan/pandas,cython-testbed/pandas,DGrady/pandas,MJuddBooth/pandas,cython-testbed/pandas,winklerand/pandas,MJuddBooth/pandas,toobaz/pandas,dsm054/pandas,toobaz/pandas,jorisvandenbossche/pandas,datapythonista/pandas,dsm054/pandas,linebp/pandas,pratapvardhan/pandas,DGrady/pandas,MJuddBooth/pandas,cbertinato/pandas,linebp/pandas,Winand/pandas,kdebrab/pandas,GuessWhoSamFoo/pandas,TomAugspurger/pandas