Fix CI, prepare test on Python 3.9 beta (#2233)

* Test on Python 3.9 beta
* Pin Sphinx
* Newer version of PyPy
diff --git a/tests/test_stl_binders.py b/tests/test_stl_binders.py
index c7b7e85..c1264c0 100644
--- a/tests/test_stl_binders.py
+++ b/tests/test_stl_binders.py
@@ -67,6 +67,7 @@
     v_int2.clear()
     assert len(v_int2) == 0
 
+
 # related to the PyPy's buffer protocol.
 @pytest.unsupported_on_pypy
 def test_vector_buffer():