pytest-copie does not recognise copier.yml, only copier.yaml

[GenevieveBuckley]

Describe the bug
The copier library accepts as input configuration files copier.yml or copier.yaml. However, pytest-copie only recognises copier.yaml, because it is hard-coded on this line:

pytest-copie/pytest_copie/plugin.py

Line 63 in bd114eb

copier_yaml = template_dir / "copier.yaml"

I think if copier accepts this as input, pytest-copie should do too.

To Reproduce
Steps to reproduce the behavior:

1. Take any currently working repository that pytest-copie can run on
2. Change the file extension of the config from .yaml to .yml, eg: mv copier.yaml copier.yml
3. Run pytest and see the error caused by pytest-copie