pip 21.3 added support to editable installs for projects that have a
pyproject.toml and use a build backend that supports PEP 660, but mypy
does not detect yet such editable installs and following errors are
reported when type checking swh modules:
Cannot find implementation or library stub for module named swh...
So temporarily pin pip until mypy properly supports PEP 660 editable
installs (see https://github.com/python/mypy/issues/12313).