Missing `py.typed` for `spdx3`

[bryceberger]

I saw that there is a py.typed file at src/spdx_tools/spdx/py.typed, but there's not a similar file at src/spdx_tools/spdx3/py.typed. From initial observation, spdx3 appears to be typed inline, the same as spdx.

[Builder106]

I want to take this — spdx3 is typed inline like spdx but is missing the PEP 561 marker, so downstream type-checkers ignore its types. I'll add src/spdx_tools/spdx3/py.typed and make sure packaging ships it.