BUG/CI/TST: fix format strings
Also test against an unsupported version of openpyxl (2.0.3)

closes #7284

repro'd the error so that's good:

https://travis-ci.org/cpcloud/pandas/jobs/26397310
 does the warning happen of openpyxl is not installed at all? (or does this not get that far)
do we test if it's not installed but running the tests anyhow (need to assert that no warning is printed)
 @jreback yes warning does happen on import of pandas ... thx for pointing that out
 should this warn on import? or when openpyxl is actually used?
 it really should warn only when used
 in that case it should throw an error because it doesn't really work with incompatible versions ...

either warn at import or raise on use
 we do this for html so it's not unprecedented
 yeah maybe just that
 well that was fun
 is their a build that doesn't have openpyxl installed at all?
 no they all have it
 could remove from one of the py3 builds
 yep take out the 3.2 maybe
 just to make sure it fails (and also does not fall back to another engine - u have to set explicitly iirc)
 @jreback ok here after squash?
 yep though maybe add a note in whatsnew 
 good call
 ok...merge when ready
 https://travis-ci.org/pydata/pandas/jobs/26564920

so i think you need to `_skip_if_no_openpyxl` in this test (before calling `openpyxl_compat.is_compat()`

numpy 1.9_master tests build doesn't have openpyxl installed at all (nor ANY excel stuff)
 Hm ok must be happening in more than one place bc I fixed one of those already. 
 Oh on
 Ok
