Provide zstd downloads

[konstin]

Currently, PyPy only provides .tar.bz2 downloads at https://downloads.python.org/pypy/. Would it be possible to provide .tar.zst downloads too? The are smaller and much faster to unpack.

I noticed in uv that installing PyPy is a lot slower than zstandard compressed CPython:

Benchmark 1: tar -xjf pypy3.11-v7.3.20-linux64.tar.bz2 -C tmp
  Time (mean ± σ):      2.246 s ±  0.339 s    [User: 2.197 s, System: 0.140 s]
  Range (min … max):    2.081 s …  2.851 s    5 runs
 
Benchmark 2: tar -xf pypy3.11-v7.3.20-linux64.tar.zst -C tmp
  Time (mean ± σ):     115.1 ms ±   2.2 ms    [User: 110.2 ms, System: 104.2 ms]
  Range (min … max):   112.6 ms … 117.3 ms    5 runs
 
Summary
  tar -xf pypy3.11-v7.3.20-linux64.tar.zst -C tmp ran
   19.50 ± 2.97 times faster than tar -xjf pypy3.11-v7.3.20-linux64.tar.bz2 -C tmp

File sizes:

• pypy3.11-v7.3.20-linux64.tar.bz2: 33M
• pypy3.11-v7.3.20-linux64.tar.zst: 29M

Reproducer:

BZ2="pypy3.11-v7.3.20-linux64.tar.bz2"
TAR="pypy3.11-v7.3.20-linux64.tar"
ZST="pypy3.11-v7.3.20-linux64.tar.zst"

wget -q --show-progress "https://downloads.python.org/pypy/$BZ2"
bzcat "$BZ2" > "$TAR"
zstd -19 -q -k "$TAR" -o "$ZST"

ls -lh "$BZ2" "$ZST" | awk '{printf "  %s: %s\n", $9, $5}'

hyperfine --warmup 1 --runs 5 \
  --prepare 'rm -rf tmp && mkdir tmp' \
  "tar -xjf $BZ2 -C tmp" \
  "tar -xf $ZST -C tmp"

rm -rf tmp

[mattip]

Hmm. I gues we could do this with minimal changes to the buildbots by changing the repacakging script used to make the release packages to build both types of packages. I would be hesitant to exclusively use zstd or change the format of the versions.json file used by the github and uv downloaders until we are sure they could support the new extension.

Something to think about if we ever do a new release.