A syntax error in the file caused the following output when trying to trigger completions in `zsh` (pressing <TAB>):
```zsh
$ fastfetch File "<stdin>", line 15
command_prefix = f"--logo-color-{i}[{flag["desc"]} ({i})]"
^^^^
SyntaxError: f-string: unmatched '['
```
This fix replaces single quotes with double quotes in the f-strings in lines 37 and 41.
Both the zsh and fish completions depend on python3.
When the completitions kick in with those shells where no python3 is
found it throws errors.
Check python3 is installed, otherwise return no completitions to avoid
throwing errors
Co-authored-by: Andrew Davis <adavis@adavis.net>
