manifest: include all .py files from hgext3rd This is required for packaging a tarball correctly on debian testing, where the current version of python and/or setuptools and/or distutils result in hgext3rd directory being included in the MANIFEST file using absolute paths to the files, and somehow that doesn't work correctly and the tarball has only a README file from topics and nothing else inside hgext3rd. Also see 3e7025a55c1b.