Tests are looking for "readthedocs" in the top source directory, but
dh_python is running tests from .pybuild/cpython3_3.*/build/.

--- a/tests/readthedocs/conftest.py
+++ b/tests/readthedocs/conftest.py
@@ -5,4 +5,4 @@ import pytest
 
 @pytest.fixture
 def docs_dir():
-    return pathlib.Path('readthedocs')
+    return pathlib.Path('../../../readthedocs')
