Use `copie_session` fixture

[sjvrijn]

Description

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• [x] This update is in line with what is recommended in the Python chapter of the Guide
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pytest-cookies had a cookies_session fixture, but pytest-copie did not yet have an equivalent when first switching from cookiecutter to copier. Since this will be added soon$^\text{tm}$, we can go back to using a dedicated fixture rather than creating such a session scoped template instance ourselves

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$^\text{tm}$ once included in pytest-copie, this PR will be marked as ready

[sjvrijn]

@egpbos good catch, pyprojroot.here references now removed.

Can't say when Soon™ is exactly, but it's active work in progress :)