Allow for .copier-answers.yaml to be default in backwards-compatible way

[timkpaine]

This is a very small backwards-compatible change.

Copier uses .copier-answers.yml as the default, and supports overriding. As discussed in #1077 .yml is the default suffix, not .yaml. It makes sense to not change the default, but perhaps it would be ok to start to check .copier-answers.yaml? This PR does that very small change, if the user has decided to use the default answers file but with .yaml suffix instead of .yml suffix, copier update will work without -a.

[sisp]

I'm open to supporting the .yaml suffix in a backwards-compatible way, but I'm afraid the solution is a little more involved.

1. See my inline comment about your suggested change.
2. The default answers file name/path is hardcoded in a few places: https://github.com/search?q=repo%3Acopier-org%2Fcopier+%22.copier-answers.yml%22+path%3A%2F%5Ecopier%5C%2F%2F&type=code This should be refactored anyway, but with a primary and secondary file name/path, we can't simply use a default value, as there isn't a single one anymore but rather some logic that first tries reading the primary file name/path and falls back to the secondary when the primary doesn't exist. Perhaps the simplest solution is to encode the default answers file name/path as None and only resolve None as .copier-answers.yml or .copier-answers.yaml in copier/_user_data.py::load_answersfile_data.

Whatever the solution looks like in the end, I have three general requests:

1. I think the .yml suffix should take precedence over the .yaml suffix for maximum backwards compatibility.
2. I think we should check for the existence of both files (.yml suffix and .yaml suffix) and raise a warning if both exist, just in case, e.g., a project was migrated from .yaml to .yml but forgot to remove the .yaml file.
3. Please add tests.

WDYT?

[sisp]

This won't work because the Path object passed to the answers_relpath argument of Subproject(local_abspath=..., answers_relpath=...) is the path of the answers file relative to local_abspath. Path(".copier-answers.yaml").exists() checks whether .copier-answers.yaml exists in the current working directory, which is not guaranteed to be local_abspath.