Fix:

```
testing/test_basics.py::TestPureChannel::test_factory_create[thread]
testing/test_basics.py::TestPureChannel::test_factory_getitem[thread]
testing/test_basics.py::TestPureChannel::test_channel_timeouterror[thread]
testing/test_basics.py::TestPureChannel::test_channel_makefile_incompatmode[thread]
  /home/ran/src/execnet/.tox/py310/lib/python3.10/site-packages/_pytest/unraisableexception.py:78: PytestUnraisableExceptionWarning: Exception ignored in: <function Channel.__del__ at 0x7fcb5b6a13f0>

  Traceback (most recent call last):
    File "/home/ran/src/execnet/execnet/gateway_base.py", line 605, in __del__
      self.gateway._send(msgcode, self.id)
  AttributeError: 'Gateway' object has no attribute '_send'. Did you mean: '_send_'?
```

`startswith` is incorrect for paths in general, e.g. doesn't handle
Windows extended-length paths and other such.

The particular code here also wasn't careful about `/a/bc` not being
relative to `/a/b`.

Use modern alternatives instead.