pip install pre-release seems to fail sporadically

[jaraco]

Running pip install -i https://devpi.net/hpk/dev/+simple/ devpi-server==2.2.0dev4 sometimes results in:

Could not find a version that satisfies the requirement devpi-common<2.1,>=2.0.6.dev0 (from devpi-server==2.2.0dev4->-r requirements.txt (line 2)) (from versions: 1.2, 2.0.0, 2.0.1, 2.0.2, 2.0.3, 2.0.4, 2.0.5, 2.0.6.dev2)

The issue appears to be in packaging, as

python -c "from packaging.specifiers import SpecifierSet; print(list(SpecifierSet('<2.1,>=2.0.6.dev0').filter(['1.2', '2.0.0', '2.0.1', '2.0.2', '2.0.3', '2.0.4', '2.0.5', '2.0.6.dev2'])))"

will return different results on different, adjacent calls.