Completions (zsh, fish): check for python3 (#1310)

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>

completions/fastfetch.fish

@@ -2,6 +2,11 @@ if not type -q fastfetch
     exit
 end
 
+command -q python3
+if test $status -ne 0
+    exit
+end
+
 complete -c fastfetch -f
 
 function __fastfetch_complete_bool

completions/fastfetch.zsh

@@ -1,6 +1,13 @@
 #compdef fastfetch
 
 function _fastfetch() {
+
+  whence python3 &> /dev/null
+  if [ $? -ne 0 ]
+  then
+    return
+  fi
+
   local state
 
   local -a opts=("${(f)$(