Allow pipx upgrade to accept multiple packages as arguments

[chrysle]

How would this feature be useful?

The possibility to handle multiple package specs at once was already introduced to the install command in #1102. The logical next step would be to extend this functionality to the pipx upgrade, and probably also pipx reinstall commands.

Describe the solution you'd like

Implement it.

Describe alternatives you've considered

Alternatively, one could write shell script or other programming code wrappers to take over this functionality, which however is not exactly user-friendly nor efficient.