move tephi to src/tephi

[ESadek-MO]

part of #164

[rcomer]

Do we know why repo review advises the code to be under src? A quick check of the big four scientific python packages show three of them (numpy, pandas, scipy) have their package at the top of the repo like tephi currently does. matplotlib has its C and C++ code under src but its python code under lib.

[ESadek-MO]

It's not repo review that's asking for this, so much as following Iris' conventions as far as I know! Repo review wants tests to be in root, but the rest of it is just standardising. If this proves too difficult, I don't mind putting this on the backburner.

[bjlittle]

@rcomer See https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/ for some pros and cons

Personally I prefer the src-layout, but that's just me 😉

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