Support arbitrary formatting helpers in FancyStrMixin

[exarkun]

FancyStrMixin supports the printf-style string formatting specification for controlling how attributes are displayed. This is handy, but printf-style only goes so far. For example, on Python 3, there is no printf-style formatter which operates on bytes instance to produce a "foo\x00"-style output (%s comes with an unwanted b prefix).

Allowing a formatting function to be supplied would let applications format things any way they wish. This is most directly needed by #6057, but is probably more widely useful.

[exarkun]

(In [35958]) Branching to 'fancyfancystrmixin-6063'

[exarkun]

(In [35960]) Branching to 'fancyfancystrmixin-6063-2'

[exarkun]

(In [35961]) Support a callable to format attribute values

refs #6063

[exarkun]

 Hope it works

[itamar]

Sigh. This is turning into an (even more) terrible API. I'm sure we can come up with a new, cleaner approach in our copious spare time. Meanwhile, this seems reasonable, please merge.

[exarkun]

(In [35964]) Merge fancyfancystrmixin-6063-2

Author: exarkun Reviewer: itamarst Fixes: #6063

Add more flexibility to the formatting done by twisted.python.util.FancyStrMixin by supporting arbitrary callables to format attribute values.