[Distutils] Are chained comparisons allowed in environment markers?

[PJ Eby]

I notice that distlib appears to support chaining of comparisons, but
the PEP does not.  ISTM that there is no sane use case for chaining,
given the nature of comparisons that are allowed.  (That is, if you
perform one equality or contains comparison between two values, then
you know how the next comparison would turn out.)

If chained comparisons are disallowed in the PEP, then distlib
probably shouldn't be allowing them either, to prevent interop issues
w/tools that only support what's in the PEP.