[master] remove __pycache__ directory with distclean

(reported by emhn: left over after a distclean)

Some of the makefiles clear that but not that one. We do
automate the distclean (via distcheck) as part of our
buildfarm. But I think this __pycache__ was introduced in python3.2 and
those build systems don't have it.

This is trivial so didn't get review.

src/lib/python/isc/bind10/Makefile.am

@@ -3,3 +3,8 @@ SUBDIRS = . tests
 python_PYTHON = __init__.py sockcreator.py component.py special_component.py \
 		socket_cache.py
 pythondir = $(pyexecdir)/isc/bind10
+
+CLEANDIRS = __pycache__
+
+clean-local:
+	rm -rf $(CLEANDIRS)