pgsql-server: Update installation docs for PL/Python to note that

Log Message:
-----------
Update installation docs for PL/Python to note that distutils is now
required. Original patch from James William Pye, editorializing by
Neil Conway.

Modified Files:
--------------
pgsql-server/doc/src/sgml:
installation.sgml (r1.203 -> r1.204)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/installation.sgml.diff?r1=1.203&r2=1.204)