TST: fix excel tests when openpyxl is not installed
TST: fix excel tests when openpyxl is not installed

Some tests fail when you don't have openpyxl installed, so explicitly
skipping those (they need it for certain exts because they write a
frame in the test)

Author: Joris Van den Bossche <jorisvandenbossche@gmail.com>

Closes #15209 from jorisvandenbossche/openpyxl-noimport and squashes the following commits:

f1cbe8f [Joris Van den Bossche] TST: fix excel tests when openpyxl is not installed

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