python: allow calling from other directories

python/setup.py.in

 import setuptools
 
+from os import path
+here = path.dirname (path.realpath (__file__))
+
 setuptools.setup(
     name='libknot',
     version='@PACKAGE_VERSION@',
@@ -9,6 +12,9 @@ setuptools.setup(
     url='https://gitlab.nic.cz/knot/knot-dns',
     license='GPL-3.0',
     packages=['libknot'],
+    package_dir = {
+        'libknot': path.join (here, 'libknot'),
+    },
     classifiers=[ # See https://pypi.org/classifiers
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Developers',