Add valgrind suppressions for python code.

Add valgrind suppressions for python code.

Python's allocator does some low-level tricks for efficiency;
unfortunately they trigger valgrind errors. Those tricks can be disabled
making instrumentation easier; but few people testing postgres will have
such a build of python. So add broad suppressions of the resulting
errors.

See also https://svn.python.org/projects/python/trunk/Misc/README.valgrind

This possibly will suppress valid errors, but without it it's basically
impossible to use valgrind with plpython code.

Author: Andres Freund
Backpatch: 9.4, where we started to maintain valgrind suppressions