pythonPackages.funcsigs: fix tests on pypy3

This package has a spurious test failure on PyPy3, which was reported
upstream a while ago:

     https://github.com/testing-cabal/funcsigs/issues/10



This is fixed thanks to the included patch, which was authored and is
also used by the Gentoo Python team.

With this, packages like 'pytest' and 'click' now work under PyPy3.

Signed-off-by: Austin Seipp <aseipp@pobox.com>