Fix pre commit config generation

[TommasoTarchi]

CI on sync-test branch of python-project-template passes, and pre-commit-config.yml is generated.

I also changed "git add *" to "git add -A" in post_gen_project.py, as suggested by @thomasisensee .

[thomasisensee]

The difference between the C++ cookiecutter and this one here is the following line:

cookiecutter-python-package/hooks/post_gen_project.py

Line 60 in de23b4c

conditional_remove(not {{ have_precommit }}, ".pre-commit-config.yaml")

This is only present in the Python cookiecutter. That is why the sync CI job on the C++ cookiecutter did create the .pre-commit-config.yaml. However, pre-commit was not run on the baked project since it is not installed. So I think explicitly installing it is a good idea.

Although ideally pre-commit should not find anything to complain about in the freshly baked project.