Warn major version difference

[yajo]

Targeting #348, from now on, if we cannot check the copier version or the installed version's major part is bigger than the template's minimal major part, there'll be proper warnings.

[codecov-io]

Codecov Report

Merging #359 (10e87b7) into master (b3190c2) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
- Coverage   97.23%   97.21%   -0.03%     
==========================================
  Files          35       35              
  Lines        2097     2115      +18     
==========================================
+ Hits         2039     2056      +17     
- Misses         58       59       +1     

Flag	Coverage Δ
unittests	97.21% <100.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
copier/errors.py	100.00% <100.00%> (ø)
copier/template.py	98.50% <100.00%> (-0.73%)	⬇️
tests/test_minimum_version.py	100.00% <100.00%> (ø)

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