add div(::Period, ::Period) method that's present in 1.8

[kleinschmidt]

1.8.1 has a method for div on time periods which uses promote:

https://github.com/JuliaLang/julia/blob/b73bf8618579d5ae0327917af590d823a100ec82/stdlib/Dates/src/periods.jl#L101

1.7.3 doesn't have this though (looks like it was added this commit and included in 1.8.0). We could add these methods in compat behind a version guard to add support for this.