LaTeX: refactor visit_enumerated_list() to use \sphinxsetlistlabels

[jfbu]

Fixes: #6511

This is only refactoring. But as a result it provides way to fix #6511. As it is not yet decided if #6511 is a "bug" or not, I did not include any addition to CHANGES file.

I hesitated to ask merge to 2.1.3 as it is not clear if bugfix. But this is refactoring which is backwards compatible.

[codecov]

Codecov Report

Merging #6533 into 2.0 will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             2.0    #6533      +/-   ##
=========================================
- Coverage   83.7%   83.67%   -0.03%     
=========================================
  Files        266      268       +2     
  Lines      40592    40624      +32     
  Branches    5979     5982       +3     
=========================================
+ Hits       33979    33994      +15     
- Misses      5291     5305      +14     
- Partials    1322     1325       +3

Impacted Files	Coverage Δ
tests/test_build_latex.py	95.33% <100%> (ø)	⬆️
sphinx/writers/latex.py	81.92% <100%> (-0.03%)	⬇️
sphinx/__init__.py	73.91% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

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[tk0miya]

LGTM!

[tk0miya]

Note: 2.1.x is a branch for urgent releases. So normal bugs is better to merge 2.0 branch.

[jfbu]

Thanks for reviewing. Rebased for merge to 2.0 branch, I will merge after testing completes.