* doc: add litgen to the automatic generators list (compiling.rst)

Added this:

[litgen]_ is an automatic python bindings generator with a focus on generating
documented and discoverable bindings: bindings will nicely reproduce the documentation
found in headers. It is is based on srcML (srcml.org), a highly scalable, multi-language
parsing tool with a developer centric approach. The API that you want to expose to python
must be C++14 compatible (but your implementation can use more modern constructs).

.. [litgen] https://pthom.github.io/litgen

* style: pre-commit fixes

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