Sphinx: custom Pygments' lexer not found

Question

I created Pygments customized lexer and style:

• acetexlexer.py (lexer file),
• acedracula.py (style file),

that work pretty well since the following command returns the expected result:

pygmentize -O style=acedracula -x -l acetexlexer.py:AceTexLexer test.tex

But I can't get them to work with Sphinx, despite extensive research on the Internet.

I tried for instance the following setup, partially based on:

• https://github.com/AdaCore/aunit/blob/v21.0.0/doc/share/ada_pygments.py
• https://github.com/AdaCore/aunit/blob/v21.0.0/doc/share/conf.py

that I found from:

https://github.com/sphinx-doc/sphinx/issues/9544

1. At the root of the source of the project, I created a _pygments subdirectory containing the lexer and style files (acetexlexer.py and acedracula.py).

2. The relevant lines of acetexlexer.py are:

    from pygments.lexer import inherit, bygroups
    from pygments.token import Text, Comment, Operator, Keyword, Name, String, \
        Number, Punctuation, Generic, Other, Whitespace
    from pygments.lexers.markup import TexLexer
   
    __all__ = ['AceTexLexer']
   
    class AceTexLexer(TexLexer):
        aliases = ['xtex', 'xlatex']
   

3. The relevant lines of my conf.py are:

    import os
    import sys
    sys.path.insert(0, os.path.abspath('.'))
    sys.path.append(os.path.abspath("./_ext"))
    sys.path.append(os.path.abspath("./_pygments"))
   
    def setup(app):
        from acetexlexer import AceTexLexer
        app.add_lexer('xlatex', acetexlexer.AceTexLexer)
   

But, when I run:

sphinx-build -v -j auto source build/html source/test.md

I get:

[...]/source/test.md:16: WARNING: Pygments lexer name 'xlatex' is not known.

What am I doing wrong?