A remnant of old code manually checks the sign of fds, but doesn't set
errno before raising OSError:
```
>>> os.dup2(-1, 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  OSError: [Errno 0] Error
```
Remove the manual check altogether: the C library functions used to
implement os.dup2() check fd validity anyway.

…6KQ4c.rst