feat: validate default values

[sisp]

I've added validation of default values which revealed an error in one of the test cases and some general weakness in the answer parsing/casting. In particular, a type: str/int/float/bool question could have None as its default value and Copier didn't complain for several reasons:

• str had a special treatment which seems obsolete to me after fix: require answer for questions without default value #958 has been merged.
• bool silently handles None where bool(None) is False.
• int and float both raise a TypeError for int(None)/float(None) which got swallowed by a too broad try/except block actually intended only for allowing type: json/yaml questions to parse answers with native (i.e. not stringified) Python values.

Note: This PR requires #958 to be merged first. The test suite fails right now but passes for me after merging #958 locally.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.52%. Comparing base (eee609d) to head (cb2e386).
Report is 429 commits behind head on master.

Files	Patch %	Lines
copier/user_data.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1075      +/-   ##
==========================================
- Coverage   96.59%   96.52%   -0.08%     
==========================================
  Files          45       45              
  Lines        3699     3710      +11     
==========================================
+ Hits         3573     3581       +8     
- Misses        126      129       +3     

Flag	Coverage Δ
unittests	96.52% <91.66%> (-0.08%)	⬇️

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[sisp]

@yajo I think this PR is ready for review now. There is one uncovered line, please see my inline comment.

[sisp]

I'm not sure how to cover this error by a test as it doesn't occur in practice because the question type is checked to be one of the keys of CAST_STR_TO_NATIVE before. Nevertheless, this function could theoretically be called in a different context, so I think it makes sense to keep the check here.

[yajo]

Could you please solve the conflicts?

[sisp]

Done.