Provide "integrations"

[pawamoy]

Is your feature request related to a problem? Please describe.
I don't like polluting my duties.py with lots of boilerplate code needed to run some tools that don't expose clean entrypoints.

For example, I'm currently playing with blacken-docs, and this is the code I have to put in my duties:

import black
from blacken_docs import format_file

def blacken_docs():
    py_md_files = set()
    for src_path in list(PY_SRC_PATHS) + [Path("docs")]:
        if src_path.is_file():
            py_md_files.add(src_path.as_posix())
        else:
            py_md_files |= set(path.as_posix() for path in src_path.rglob("*.py"))
            py_md_files |= set(path.as_posix() for path in src_path.rglob("*.md"))

    black_mode = black.Mode()
    retv = 0
    for filename in sorted(py_md_files):
        retv |= format_file(filename, black_mode, skip_errors=False, rst_literal_blocks=False)
    return retv

ctx.run(blacken_docs, title="Formatting code blocks")

See other examples here: pawamoy/copier-pdm#21.

Describe the solution you'd like
I wonder if duty couldn't provide an integrations subpackage that offers Python callables to run many different tools and their commands.

For example, I'd like to import a function blacken_docs from duty.integrations.blacken_docs that accepts lists of paths to check, recursively, with a list of extensions to pick up maybe, and probably other options (that just cater to my own needs first).

Describe alternatives you've considered
Copy/paste, or inclusion in my copier-pdm template (which is kinda copy/paste anyway).

Additional context
The goal is re-usability and performance. We don't want to run subprocesses, or have argparse parse arguments again and again.

Current integrations I'd like to have:

• flake8 check (critical)
• mkdocs build strict (critical)
• mypy check (critical)
• safety check
• blacken-docs