Add default pip completion

The default pip completion is suboptimal because it uses the old Zsh
completion engine which is compatible with bash.

zshrc

@@ -41,6 +41,19 @@ unsetopt flow_control && stty -ixon
 autoload -U compinit
 compinit
 
+# Add pip to the old completion engine if present
+if which pip &> /dev/null; then
+  function _pip_completion {
+    local words cword
+    read -Ac words
+    read -cn cword
+    reply=( $( COMP_WORDS="$words[*]" \
+               COMP_CWORD=$(( cword-1 )) \
+               PIP_AUTO_COMPLETE=1 $words[1] ) )
+  }
+  compctl -K _pip_completion pip
+fi
+
 # Enable prompt themes
 autoload -Uz promptinit
 promptinit