setup: use absolute paths

This allows running the setup.py script from another directory. It is
required because pybuild runs tests into a temp dir which is a sub
folder of the sources.

Signed-off-by: Robin Jarry <robin@jarry.cc>

Gbp-Pq: Name setup-use-absolute-paths.patch

setup.py

@@ -95,8 +95,12 @@ else:
         )
 
 
+def abspath(rel):
+    return os.path.join(os.path.dirname(__file__), rel)
+
+
 def get_greenlet_version():
-    with open('src/greenlet/__init__.py') as f:
+    with open(abspath('src/greenlet/__init__.py')) as f:
         looking_for = '__version__ = \''
         for line in f:
             if line.startswith(looking_for):
@@ -109,7 +113,7 @@ setup(
     name="greenlet",
     version=get_greenlet_version(),
     description='Lightweight in-process concurrent programming',
-    long_description=readfile("README.rst"),
+    long_description=readfile(abspath("README.rst")),
     long_description_content_type="text/x-rst",
     url="https://greenlet.readthedocs.io/",
     keywords="greenlet coroutine concurrency threads cooperative",