RST: python3 language records rendered as plain text.

[penguinolog]

Pygments declares .. code-block:: python3 as language key for python 3.x code.
This is correctly rendered by docutils, but not recognized by GitHub.

Code example:

.. code-block:: python3

    async def somefunc():
        return await some_coroutine()

[kivikakk]

We use Linguist to perform syntax highlighting on GitHub, including in code blocks — you'll want to open a PR to that to add an alias (like "python3") for Python here, or else just use python.

Even though we use docutils to render rST, we don't use Pygments for syntax highlighting.