Update the hooks used in the documentation and tests

[mwouts]

The CI is failing on the test test_pre_commit_hook_sync_black_nbstripout, see also #940 and #942

I cannot reproduce that error locally, so I am trying to update the hooks used by this test.

The details for the error are below:

    @skip_pre_commit_tests_on_windows
    @skip_pre_commit_tests_when_jupytext_folder_is_not_a_git_repo
    def test_pre_commit_hook_sync_black_nbstripout(
        tmpdir,
        cwd_tmpdir,
        tmp_repo,
        jupytext_repo_root,
        jupytext_repo_rev,
        notebook_with_outputs,
    ):
        """Here we sync the ipynb notebook with a py:percent file and also apply black and nbstripout."""
        pre_commit_config_yaml = f"""
    repos:
    - repo: {jupytext_repo_root}
      rev: {jupytext_repo_rev}
      hooks:
      - id: jupytext
        args: [--sync, --pipe, black]
        additional_dependencies:
        - black==20.8b1  # Matches hook
    
    - repo: https://github.com/psf/black
      rev: 20.8b1
      hooks:
      - id: black
    
    - repo: https://github.com/kynan/nbstripout
      rev: 0.3.9
      hooks:
      - id: nbstripout
    """
        tmpdir.join(".pre-commit-config.yaml").write(pre_commit_config_yaml)
    
        tmp_repo.git.add(".pre-commit-config.yaml")
        pre_commit(["install", "--install-hooks", "-f"])
    
        tmpdir.join(".jupytext.toml").write('formats = "ipynb,py:percent"')
        tmp_repo.git.add(".jupytext.toml")
        tmp_repo.index.commit("pair notebooks")
    
        # write a test notebook
        write(notebook_with_outputs, "test.ipynb")
    
        # try to commit it, should fail because
        # 1. the py version hasn't been added
        # 2. the first cell is '1+1' which is not black compliant
        # 3. the notebook has outputs
        tmp_repo.git.add("test.ipynb")
        with pytest.raises(
            HookExecutionError,
            match="files were modified by this hook",
        ):
            tmp_repo.index.commit("failing")
    
        # Add the two files
        tmp_repo.git.add("test.ipynb")
>       tmp_repo.git.add("test.py")

 >           raise GitCommandError(redacted_command, status, stderr_value, stdout_value)
E           git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
E             cmdline: git add test.py
E             stderr: 'fatal: pathspec 'test.py' did not match any files'

[codecov]

Codecov Report

Merging #943 (a3cb07b) into main (adff5a2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #943   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files         117      117           
  Lines       10869    10869           
=======================================
  Hits        10772    10772           
  Misses         97       97           

Impacted Files	Coverage Δ
tests/test_pre_commit_2_sync_nbstripout.py	100.00% <ø> (ø)
tests/test_pre_commit_3_sync_black_nbstripout.py	100.00% <ø> (ø)
tests/test_pre_commit_5_reformat_markdown.py	100.00% <ø> (ø)

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