Replace Python code-blocks with double colons

[radarhere]

#6754 has pointed out that sometimes we use .. code-block:: python and sometimes we use ::.

Because :: is rendered as a single colon, perhaps .. code-block:: python is used where we don't want to show a colon to the reader. This PR replaces .. code-block:: python with :: if it is already following a colon, making the documentation syntax consistent.

[hugovk]

We can also use :: in other cases too, when there is no colon.

For example:

Some header
-----------

::

    a = 1

Some more text. ::

    a = 1

Renders as:

[radarhere]

Ok, I've replaced all instances of .. code-block:: python