[banquepopulaire] Backport support for convert virtual method in SplitKeyboard

In core 1.5, SplitKeyboard.checksum does not call convert.
The banquepopulaire plugin requires it, so adding support in compat module

Fix #271
Status Job ID Name Coverage
  Build
created #19441
build:2.7
created #19442
build:3
 
  Test
created #19444
allowed to fail
lint:2.7
created #19448
allowed to fail
lint:3
created #19443
pyflakes:2.7
created #19447
pyflakes:3
created #19445
unittests:2.7
created #19449
unittests:3
created #19446
allowed to fail
unittests-modules:2.7
created #19450
allowed to fail
unittests-modules:3
 
  Deploy
created #19451
doc:2.7
created #19452
doc:3