redirect_stdio is broken by Julia 1.7

[stevengj]

As described in #968 (comment) and #968 (comment), the stdio redirection code will need to change for Julia 1.7 to use the new mechanism from JuliaLang/julia#39132 — currently, starting the IJulia kernel in Julia 1.7 gives an error:

ERROR: LoadError: ArgumentError: expecting stdout stream
Stacktrace:
 [1] (::Base.redirect_stdio)(io::IJulia.IJuliaStdio{Base.PipeEndpoint})
   @ IJulia ~/.julia/dev/IJulia/src/stdio.jl:30
 [2] init(args::Vector{String})
   @ IJulia ~/.julia/dev/IJulia/src/init.jl:109
 [3] top-level scope
   @ ~/.julia/dev/IJulia/src/kernel.jl:24
in expression starting at /Users/stevenj/.julia/dev/IJulia/src/kernel.jl:24

[stevengj]

(Seems to work fine in Julia 1.6.)