Module no longer builds on Python 3.14 due to slimit dependency

[mafr]

When I try to install the webassets dependencies locally with Python 3.14, that no longer works because the 2to3 tool has been removed and slimit requires that ("uv sync" aborts with the message "Python 3.X support requires the 2to3 tool.")

The slimit tool hasn't been updated in 7 years, but there is a fork that dropped Python 2 support and seems to work.