bpo-18174: Fix fd_count() on FreeBSD (GH-7420)

Python 2.7 doesn't have FileNotFoundError exception: use OSError and
errno.ENOENT.

Lib/test/support/__init__.py

@@ -2069,8 +2069,9 @@ def fd_count():
         try:
             names = os.listdir("/proc/self/fd")
             return len(names)
-        except FileNotFoundError:
-            pass
+        except OSError as exc:
+            if exc.errno != errno.ENOENT:
+                raise
 
     old_modes = None
     if sys.platform == 'win32':