[3.13] gh-112346: Always set OS byte to 255, simpler gzip.compress function. (GH-120486) (#120563)

gh-112346: Always set OS byte to 255, simpler gzip.compress function. (GH-120486)

This matches the output behavior in 3.10 and earlier; the optimization in 3.11 allowed the zlib library's "os" value to be filled in instead in the circumstance when mtime was 0.  this keeps things consistent.
(cherry picked from commit 08d09cf5ba041c9c5c3860200b56bab66fd44a23)

Co-authored-by: Ruben Vorderman <r.h.p.vorderman@lumc.nl>