Add a skeleton of the pytest plugin

[nedtwigg]

• First step towards pytest-selfie initial acceptance tests selfie#296

[nedtwigg]

Currently broken because there are no tests.

[nedtwigg]

From this list it looks like we should name the project pytest-selfie to follow the naming convention.

[nedtwigg]

How I did this (roughly):

• use this project to setup the template
  • https://github.com/pytest-dev/cookiecutter-pytest-plugin
• convert it to poetry
• this blog post was helpful: https://blog.pecar.me/pytest-plugin
• docs on writing plugins: https://docs.pytest.org/en/7.1.x/how-to/writing_plugins.html
• docs on writing hook functions: https://docs.pytest.org/en/7.1.x/how-to/writing_hook_functions.html

In the end we have:

• pytest-selfie/pytest_selfie/plugin.py
  • pytest_sessionstart - called before all tests
  • pytest_sessionfinish - called after all tests
  • pytest_pyfunc_call - called before test, yield, after the test
• example-pytest-selfie - playground where we can use pytest-selfie like an end user