plan for a change in broadcasting rules in Julia 1.7

[bkamins]

This is a transition PR clearly communicating to users what will happen in DataFrames.jl 1.4 release, see #2804.

Since the change planned in DataFrames.jl 1.4 will be mildly breaking and will introduce an inconsistency between Julia versions an additional transition period is I think a good approach.

[bkamins]

For a reference. The current behavior is:

julia> df = DataFrame(x=1:3)
3×1 DataFrame
 Row │ x
     │ Int64
─────┼───────
   1 │     1
   2 │     2
   3 │     3

julia> df.x .= 'x'
3-element Vector{Int64}:
 120
 120
 120

julia> df # people typically expect that `:x` column contains 'x' Char values after the previous operation
3×1 DataFrame
 Row │ x
     │ Int64
─────┼───────
   1 │   120
   2 │   120
   3 │   120

julia> df.x .= 1.5 # people typically expect this operation to work
ERROR: InexactError: Int64(1.5)

and after the change both operations will produce the expected result described in the comment under Julia 1.7 and newer. Under Julia 1.6 and older they will remain as they are now as it is impossible to fix this behavior there.

[bkamins]

this is what I would recommend.

[bkamins]

Thank you!