It allows to run unasync on multiple files using multiples rules if needed, without being tied to setuptools.

@sethmlarson Now that you taught me that using Rule.unasync_files directly made sense, I'm not sure if this is such a good idea. Right now, I have a proof-of-concept that works in hip using this API:

import unasync

unasync.unasync_files(
    [
        "test/with_dummyserver/sync/__init__.py",
        "test/with_dummyserver/async/test_poolmanager.py",
    ],
    rules=[
        unasync.Rule(
            "test/with_dummyserver/async",
            "test/with_dummyserver/sync",
            replacements={
                "AsyncPoolManager": "PoolManager",
                "test_all_backends": "test_sync_backend",
            },
        )
    ],
)

Since I have only a single rule, I guess I could use Rule.unasync_file and loop over files. This pull request feels more general, and since I already had the code, I submitted it. Please tell me what you think.