The shinyapps.io service has Python 3.8 on the system, per the admin guide. If you use the reticulate package, you should be able to install any Python package that doesn't have additional systems dependencies, as long as you use the right name.
For an example of using `py_install()` to install a Python package, see this Community post.
It is not currently possible to install a Python package that has additional systems dependencies, but you could file an issue in the shinyapps.io package dependencies repo if you wish to get one added in the future.
If you are looking for instructions on using shiny for Python on shinyapps.io, you can find instructions in the user guide.