Fix documentation for readline license (#1297)

NITS: I fixed the license for readline in the documentation to GPLv3.

cf. https://tiswww.case.edu/php/chet/readline/rltop.html

> Readline is free software, distributed under the terms of the [GNU
General Public License, version
3](http://www.gnu.org/licenses/gpl.html).

cf.
https://gregoryszorc.com/docs/python-build-standalone/main/running.html#licensing

> Most licenses are fairly permissive. Notable exceptions to this are
GDBM and readline, which are both licensed under GPL Version 3.

docs/guide/toolchains/cpython.md

@@ -28,7 +28,7 @@ different from a regular Python build.
 
 The following changes to a regular Python versions you should be aware of:
 
-* `libedit` instead of `readline`: unfortunately `readline` is GPL2 licensed
+* `libedit` instead of `readline`: unfortunately `readline` is GPLv3 licensed
   and this is a hazard for redistributions.  As such, the portable Python
   builds link against the more freely licensed `libedit` instead.