Return True instead of None to avoid cachefunc being useless

[user202729]

Just a trivial fix.

(probably just the tip of the iceberg though.)

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[mantepse]

I don't understand the remark:

Note that :func:cached_method does not work with methods returning None, so True is returned instead.

In what sense, and why?

[user202729]

@mantepse It's explained in the documentation

cdef class CachedMethodCallerNoArgs(CachedFunction):
    """
    Utility class that is used by :class:`CachedMethod` to bind a
    cached method to an instance, in the case of a method that does
    not accept any arguments except ``self``.

    .. NOTE::

        The return value ``None`` would not be cached. So, if you have
        a method that does not accept arguments and may return ``None``
        after a lengthy computation, then ``@cached_method`` should not
        be used.

In practice, what happens is the thing will just be recomputed each time.

(I have a pending PR that adds a debug option to find all cases where the thing returns None. Apparently a lot of Sage code does that — small percentage, but still a lot.)

[mantepse]

Looks good! Thank's for catching!