Add -B to python invocation to avoid generating pyc files

Without -B, python invocations may result in generation of pyc files
for modules within the source tree, which does not work well when the
source tree is read-only.

2016-12-17  Martin Galvan  <martingalvan@sourceware.org>

	* Rules (python-flags, python-invoke): New.
	($(test-printers-out)): Use $(python-flags).