[python mode] Accept underscores in number literals
[python mode] Accept underscores in number literals

Closes #4506

MarcelGerber/CodeMirror,pabloferz/CodeMirror,MarcelGerber/CodeMirror,ejgallego/CodeMirror,ficristo/CodeMirror,crazy4chrissi/CodeMirror,grzegorzmazur/CodeMirror,nightwing/CodeMirror,ejgallego/CodeMirror,hackmdio/CodeMirror,pabloferz/CodeMirror,nightwing/CodeMirror,thomasjm/CodeMirror,ficristo/CodeMirror,schanzer/CodeMirror,mtaran-google/CodeMirror,codio/CodeMirror,jayaprabhakar/CodeMirror,jayaprabhakar/CodeMirror,kerabromsmu/CodeMirror,notepadqq/CodeMirror,notepadqq/CodeMirror,grzegorzmazur/CodeMirror,dperetti/CodeMirror,jkaplon/CodeMirror,dperetti/CodeMirror,codio/CodeMirror,wingify/CodeMirror,ejgallego/CodeMirror,schanzer/CodeMirror,mtaran-google/CodeMirror,namin/CodeMirror,thomasjm/CodeMirror,grzegorzmazur/CodeMirror,jayaprabhakar/CodeMirror,jkaplon/CodeMirror,crazy4chrissi/CodeMirror,ficristo/CodeMirror,namin/CodeMirror,SidPresse/CodeMirror,kerabromsmu/CodeMirror,wingify/CodeMirror,thomasjm/CodeMirror,mtaran-google/CodeMirror,jkaplon/CodeMirror,schanzer/CodeMirror,pabloferz/CodeMirror,nightwing/CodeMirror,rrandom/CodeMirror,wingify/CodeMirror,kerabromsmu/CodeMirror,hackmdio/CodeMirror,SidPresse/CodeMirror,rrandom/CodeMirror,codio/CodeMirror,SidPresse/CodeMirror,MarcelGerber/CodeMirror,crazy4chrissi/CodeMirror,dperetti/CodeMirror,namin/CodeMirror,rrandom/CodeMirror,notepadqq/CodeMirror,hackmdio/CodeMirror