Fix python-mode hideshow regexp

Fix python-mode hideshow regexp

2015-02-07 "Fix hideshow integration[...]" changed the regexp added to
`hs-special-modes-alist' so that it worked when not searching from the
beginning of the line.  However, this allows matching tokens ending in
"def" or "class", not just those keywords.  This results in an infinite
loop in hs-hide-all (Bug #24815).

* lisp/progmodes/python.el (python-mode): Add symbol boundaries around
the def|class matching part of the regexp added to
hs-special-modes-alist.