bin/install: Temporarily pin pip to < 21.3
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).
Migrated from D8249 (view on Phabricator)