feat: Parse templates in _answers_file setting

[icj217]

Adds the ability for _answers_file to contain template variables. For example:

# copier.yml
_answers_file: ".copier-answers.{{ foo }}.yml"

foo:
  type: str

Closes #866

Since #882 seems to have been abandoned and the tests weren't actually working, I've opened up this PR instead.

[nilsdebruin]

Great! Would be nice if this feature is added!

[sisp]

I have a couple of suggestions for improvement. But overall it looks good to me. 🙂

[icj217]

@yajo Would you mind taking a look and approving the workflow runs?

[icj217]

@sisp Are you able to trigger the workflow runs for this PR?

[sisp]

I don't think I can help because I have no permissions for this project.

[icj217]

@yajo Sorry for the bump but wondering if you can approve these workflow runs?

[icj217]

@yajo Any concerns with letting the workflows run?

[yajo]

Triggered! No concerns, just I had 120 emails from copier 😂

[yajo]

Pretty clean, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.36%. Comparing base (005a897) to head (5e9d7d7).
Report is 569 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
- Coverage   96.56%   96.36%   -0.20%     
==========================================
  Files          41       43       +2     
  Lines        3054     3247     +193     
==========================================
+ Hits         2949     3129     +180     
- Misses        105      118      +13     

Flag	Coverage Δ
unittests	96.36% <100.00%> (-0.20%)	⬇️

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