LaTeX: use lookbehind regex to not match semicolon at start

[jfbu]

Closes: #5576

Here is screen shot of a portion of the index in library.pdf from CPython 3.7 documentation, showing indexing of ; now works:

I used a look-behind regex as advocated at this stackoverflow answer

Relates

• make all-pdf fails invocation of xindy #5561

[codecov]

Codecov Report

Merging #5577 into 1.8 will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              1.8    #5577      +/-   ##
==========================================
- Coverage   82.05%   82.05%   -0.01%     
==========================================
  Files         306      300       -6     
  Lines       40388    40049     -339     
  Branches     6241     6187      -54     
==========================================
- Hits        33141    32862     -279     
+ Misses       5862     5809      -53     
+ Partials     1385     1378       -7

Impacted Files	Coverage Δ
sphinx/writers/latex.py	84.06% <100%> (ø)	⬆️
sphinx/make_mode.py
sphinx/search/__init__.py
sphinx/quickstart.py
sphinx/errors.py
sphinx/__init__.py
sphinx/apidoc.py

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[jfbu]

@tk0miya thanks a lot for quick review. I will soon make however another PR to fix #5579 and it fixes the #5576 issue as well. But it is less minimal change.

[jfbu]

This is closed without merging because it got superseded by #5583.