Fix version pinning for zstandard on pypy3.9

[yunzheng]

Ordering matters to ensure that the pinned version takes precedence

Fixes #185

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.03%. Comparing base (04aec16) to head (0f744da).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
+ Coverage   82.98%   83.03%   +0.05%     
==========================================
  Files          34       34              
  Lines        3602     3602              
==========================================
+ Hits         2989     2991       +2     
+ Misses        613      611       -2     

Flag	Coverage Δ
unittests	83.03% <ø> (+0.05%)	⬆️

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[yunzheng]

Looks to be an issue with tox, it doesn't respect the order and will sort it before passing to pip like:

pypy3: 3861 W install_package_deps> python -I -m pip install lz4 'msgpack>=0.5.2' 'tzdata; platform_system == "Windows"' zstandard 'zstandard==0.23.0; platform_python_implementation == "PyPy" and python_version == "3.9"' [tox/tox_env/api.py:461]

If the bare zstandard comes after the one with the constraint, it would work.

[Schamper]

Luckily we're dropping 3.9 support soon.