Our build scripts invoke `python3` in preference to `python`, so make the
default instruction use `python3`. On many systems (macOS, some Linux),
`python` invokes Python 2 which our scripts do not support.

Suggest --user by default. It's usually the right thing outside of venvs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>