LaTeX: use \small fontsize in code-blocks with xelatex and lualatex

[jfbu]

Amusingly the character width of FreeFont Mono (now used by xelatex/lualatex) is quite large, a bit like Courier compared to Times, which is used by pdflatex. Thus in my testing it gives better looking result to use \small with these Unicode engines too. Of course if user changes the font setting, it may be needed that he/she uses also 'fvset' key of latex_elements to adjust.

Relates

• LaTeX: support for Greek and Cyrillic #5645
• Let LaTeX with XeTeX/LuaTeX use normal sized typeface in code-blocks #4793

[codecov]

Codecov Report

Merging #5768 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5768      +/-   ##
==========================================
- Coverage   83.27%   83.24%   -0.03%     
==========================================
  Files         294      296       +2     
  Lines       39577    39614      +37     
  Branches     5878     5881       +3     
==========================================
+ Hits        32959    32978      +19     
- Misses       5257     5272      +15     
- Partials     1361     1364       +3

Impacted Files	Coverage Δ
sphinx/writers/latex.py	83.28% <ø> (ø)	⬆️
sphinx/__init__.py	76% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

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