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.