This adds a new flag, -p, to %pyproject_buildrequires.
When set, the runtime dependencies are read from the pyproject.toml's
[project] table.
See: https://bugzilla.redhat.com/2261939
pyproject_buildrequires.py already had a short `-p` option for
--python3_pkgversion (hidden from the macro users).
This change removes the one-letter option and leaves the long-one.
`-p` is now reused for reading dependencies from pyproject.toml
and made visible to the macro users.
