Enable CompatHelper again

[JoshuaLampert]

While checking why I cannot instantiate PrettyTables.jl v3 with packages from the SciML ecosystem I recognized it is because SymbolicIndexingInterface.jl doesn't allow PrettyTables.jl v3 (which is of course ok because PrettyTables.jl v3 has only been released some hours ago), but since CompatHelper is disabled due to inactivity in this repo, you won't get a CompatHelper PR soon. Therefore, I would like to request re-enabling CompatHelper again (takes ~5 seconds).

[AayushSabharwal]

Thanks for noticing, I re-enabled it.

[JoshuaLampert]

Thanks!