Exclude _dev directory from pytest

[slizewsk]

Added norecursedirs to exclude _dev directory from pytest in pyproject.toml.

Specifically, only addition is

norecursedirs = ["{{ cookiecutter.module_name }}[\\/]_dev"] 

under [tool.pytest.ini_options]

This addition ensures pytest never tries to go into the /_dev folder.
Fixes #82

Hope this is a flexible solution 👍

[mhvk]

@Cadair - this fix helped in my class, where students set up their own packages using the package template. Note that there is another fix in #84, which I think as a fix is less good, but the changes to the text there seem to make sense.

[Cadair]

Thanks!