Fix saving of DXF files (python3)

- Python3 does not allow to write strings to a file opened as "wb"; it
  expects bytes instead. Since ASCII DXF files are written using strings,
  the file should be opened as "w". Should be ok for python2 as well.
- See also issue 003862, https://tracker.freecadweb.org/view.php?id=3862

src/Mod/Draft/importDXF.py

@@ -3932,7 +3932,7 @@ def exportPage(page, filename):
     c = dxfcounter()
     pat = re.compile("(_handle_)")
     template = pat.sub(c.incr, template)
-    f = pythonopen(filename, "wb")
+    f = pythonopen(filename, "w")
     f.write(template)
     f.close()