No support for PowerShell 7 (PowerShell Core)

```
> python examples\click-completion-command install
Error: pwsh is not supported.
```
```
> powershell
Windows PowerShell
[...]
PS > python examples\click-completion-command install
powershell completion installed in b'C:\\Users\\thorsten\\Documents\\WindowsPowerShell\\Microsoft.PowerShell_profile.ps1'
```