hgbuildbot.py throws No module named 'future'

[Zash]

$ hg push -fr.
pushing to ssh://hg@hg.example.org/repo
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 1 changes to 1 files
remote: 2 new obsolescence markers
remote: loading changegroup.buildbot hook failed:
remote: abort: No module named 'future'
abort: stream ended unexpectedly (got 0 bytes, expected 4)

Server has Mercurial 5.7 on Debian 10

hg debuginstall from server:

checking encoding (UTF-8)...
checking Python executable (/usr/bin/python3)
checking Python implementation (CPython)
checking Python version (3.7.3)
checking Python lib (/usr/lib/python3.7)...
checking Python security support (sni,tls1.0,tls1.1,tls1.2)
checking Rust extensions (missing)
checking Mercurial version (5.7)
checking Mercurial custom build ()
checking module policy (c)
checking installed modules (/usr/lib/python3/dist-packages/mercurial)...
checking registered compression engines (bz2, bz2truncated, none, zlib, zstd)
checking available compression engines (bz2, bz2truncated, none, zlib, zstd)
checking available compression engines for wire protocol (zstd, zlib, bz2, none)
checking "re2" regexp engine (missing)
checking templates (/usr/lib/python3/dist-packages/mercurial/templates)...
checking default template (/usr/lib/python3/dist-packages/mercurial/templates/map-cmdline.default)