[FIX][report_xls] Fix regression introduced in 65223f4.
[FIX][report_xls] Fix regression introduced in 65223f4.

Variables are now at module level because they have to be mocked when there is
an `ImportError`, so we got an `AttributeError` when trying to use them.

ddico/reporting-engine,acsone/reporting-engine,syci/reporting-engine,acsone/reporting-engine,ddico/reporting-engine,open-synergy/reporting-engine,syci/reporting-engine,diagramsoftware/reporting-engine,eezee-it/reporting-engine,open-synergy/reporting-engine,acsone/reporting-engine,eezee-it/reporting-engine,diagramsoftware/reporting-engine,diagramsoftware/reporting-engine,open-synergy/reporting-engine,brain-tec/reporting-engine,brain-tec/reporting-engine,open-synergy/reporting-engine,brain-tec/reporting-engine,eezee-it/reporting-engine,ddico/reporting-engine,diagramsoftware/reporting-engine,ddico/reporting-engine,brain-tec/reporting-engine,acsone/reporting-engine,eezee-it/reporting-engine