Check if readline.__doc__ is None (#29)

## Changes
In certain Windows installs, readline.__doc__ is None. Added a check
before iterating.
## Issue
Improving Windows compatibility #9 
## Related pull request
Add compatibility for Windows #14 
## Inspiration
[Add handling for Windows "readline.__doc__ is NoneType"
error](clarkperkins/click-shell#11)

---------

Co-authored-by: Mariusz Kupidura <2480867+fwkz@users.noreply.github.com>

riposte/riposte.py

@@ -13,7 +13,7 @@
 
 
 def is_libedit():
-    return "libedit" in readline.__doc__
+    return readline.__doc__ and "libedit" in readline.__doc__
 
 
 class Riposte(PrinterMixin):