fix config scope and tmp dir creation

[sjvrijn]

See #89 (comment)

[12rambau]

how will it continue to work at function scope if the config file fixture is session scoped ?

[sjvrijn]

Setting the config scope to session means it's only generated once per session, i.e., the result is reused for all subsequent calls. This seems to work fine for pytest-cookies: https://github.com/hackebrot/pytest-cookies/blob/28492af5bba1da3a2bed19389f96584e51c1e08f/src/pytest_cookies/plugin.py#L119

[sjvrijn]

@12rambau, are there any other questions you have about this, or is there anything else I can do?

[12rambau]

I just need to find time to perform my OSS duties, I like this repository and use it a lot I'll let you know if anything is not working.