Make tests compatible with pyftpdlib 2.0

[musicinmybrain]

Type of changes

• Tests

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate. Is this needed?
• I've added tests for new code. N/A, testing fixes
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

Inspired by @mweinelt via #591.

• Updates the pyftpdlib test dependency from ~=1.5 to ~=2.0
• Uses pyftpdlib’s test extra (introduced in 2.0) to bring in dependencies for pyftpdlib.test, rather than naming them and maintaining their version bounds manually.
• Follow the renaming of ThreadedTestFTPd to FtpdThreadWrapper

---

I tested this locally in a Python 3.13 virtualenv, first applying #570 and #587. I do find that I can sometimes get all the tests to pass, but there are almost always some flaky failures where it seems like the test FTP server is not ready, like:

====================================== short test summary info =======================================
FAILED tests/test_ftpfs.py::TestFTPFS::test_download_4 - fs.errors.RemoteConnectionError: unable to connect to 127.0.0.1
FAILED tests/test_ftpfs.py::TestFTPFSNoMLSD::test_download_0 - fs.errors.RemoteConnectionError: unable to connect to 127.0.0.1
====================== 2 failed, 2652 passed, 26 skipped, 46 warnings in 38.61s ======================

I have not tried to understand or fix that.