sh: Add short py alias for repl

Added `py` alias for any of `ptipython`, `ipython`, `python` found in
that order.
If python is not installed, will not create the alias.

sh/.config/sh/alias

@@ -92,3 +92,12 @@ if exist vifm; then
     alias vm=vifm
     alias vmm='vifm ${PWD}'
 fi
+
+# python
+if exist ptipython; then
+    alias py=ptipython
+elif exist ipython; then
+    alias py=ipython
+elif exist python; then
+    alias py=python
+fi