mypy: replace fchmod by chmod

The former is unix-only, which complains when running on the Windows
platform -- even though this code is in a unix-specific branch.

But per the docs:

> As of Python 3.3, this is equivalent to os.chmod(fd, mode).

So we can just use the identical function that is available in more
places.

mesonbuild/scripts/depfixer.py

@@ -154,7 +154,7 @@ def open_bf(self, bfile: str) -> None:
     def close_bf(self) -> None:
         if self.bf is not None:
             if self.bf_perms is not None:
-                os.fchmod(self.bf.fileno(), self.bf_perms)
+                os.chmod(self.bf.fileno(), self.bf_perms)
                 self.bf_perms = None
             self.bf.close()
             self.bf = None