spack python: allow use of IPython (#20329)

This adds a -i option to "spack python" which allows use of the
IPython interpreter; it can be used with "spack python -i ipython".
This assumes it is available in the Python instance used to run
Spack (i.e. that you can "import IPython").

share/spack/spack-completion.bash

@@ -1373,7 +1373,7 @@ _spack_pydoc() {
 _spack_python() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -V --version -c -m"
+        SPACK_COMPREPLY="-h --help -V --version -c -i -m"
     else
         SPACK_COMPREPLY=""
     fi