Remove @mark.fails_on_windows to enable Windows test support

[Spitfire1900]

Add platform detection for Windows/Unix command differences and skip chmod/chown permission checks on Windows where these aren't supported.

[carletes]

Thanks, @Spitfire1900! Did you have a chance of testing this on a Windows system? I do not have any around, so I cannot do that.

Do you know whether they can be selected on Github Actions, by the way?

[Spitfire1900]

Despite passing in GH actions attempting to reproduce the pass locally resulted in issues. This is not ready to be merged.

FAILED test_server.py::test_ssh_failed_commands - AssertionError: assert 'Access is denied.\r\n' == 'The process ... process.\r\n'
FAILED test_sftp.py::test_symlink - OSError: Failure
FAILED test_sftp.py::test_lstat - OSError: [WinError 1314] A required privilege is not held by the client: 'C:\\Users\\XXXXXXXX\\AppData\\Local\\Temp\\tmpvxwelqfp\\foo' -> 'C:\\Users\\XXXXXXXX\\AppData\\Local\\Temp\\tmpvxwelqfp\\...

.venv\Lib\site-packages\paramiko\pkey.py:735: SSHException
========================================================================================= warnings summary ========================================================================================== 
test_mock.py:8
  C:\TMP\ssh_mock\test_mock.py:8: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
  Use @pytest.fixture instead; they are the same.
    @yield_fixture()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
====================================================================================== short test summary info ====================================================================================== 
ERROR test_mock.py::test_ssh_session - paramiko.ssh_exception.SSHException: unpack requires a buffer of 4 bytes