Move show-fixtures code from python.py to fixtures.py

[bluetech]

It makes more sense, also, we have a long term idea of generalizing fixture support to items defined by other plugins, not just python, in which case --fixtures would definitely not be python-plugin specific.

[WarrenTheRabbit]

@bluetech Moving the code to fixtures.py sounds great to me, and your remark about generalising fixture support is very interesting.

I recently tried to resolve an issue with #12129. @The-Compiler floated the idea originally in #11295 and I give three justifications in my PR description (and one comment) for making the change.

But in short, I was working in python.py. Intuitively, I would have preferred to be in fixtures.py, and in fact I had to import a function from there. I was wanting to make a separate proposal to move the code into fixtures.py, or indeed to ask what the rules are around breaking out content from large files into specialised subfiles. e.g. parts of fixtures.py into show_fixtures.py, for instance.

Also, greetings to all. I hope everyone has been well. Hopefully I have now returned and can be more involved; assuming, in (my recollection of) @RonnyPfannschmidt's words, I'm not put through another bad fate compression algorithm.