Bump to a more modern version of Sphinx

However, don't use 4.1, as it will need a new release of breathe to work
(e.g. see breathe-doc/breathe#711).

doc/py-items.rst

@@ -15,10 +15,10 @@ receiving).
    .. automethod:: dynamic_shape
    .. automethod:: compatible_shape
 
-.. autoclass:: spead2.Item(\*args, \*\*kwargs, value=None)
+.. autoclass:: spead2.Item(*args, **kwargs, value=None)
 
    .. autoattribute:: value
-   .. autoinstanceattribute:: version
+   .. autoattribute:: version
       :annotation:
 
 .. autoclass:: spead2.ItemGroup

doc/py-thread-pools.rst

@@ -16,8 +16,6 @@ pool with only one thread.
 .. [#] This is a limitation of the current design that will hopefully be
    overcome in future versions.
 
-.. py:currentmodule:: spead2
-
 .. py:class:: spead2.ThreadPool(threads=1, affinity=[])
 
    Construct a thread pool and start the threads. A list of integers can be

requirements-readthedocs.txt

@@ -1,3 +1,3 @@
-sphinx==3.0.3
-breathe==4.18.0
+sphinx==4.0.3
+breathe==4.30.0
 -r requirements.txt