Merge remote-tracking branch 'mmn/file_and_io_handling_diff' into develop

* ADD patch by Mikael Nordfeldth to file python3 syntax compatibility due to
   the 'file' builtin being removed.
       isinstance(foo, file) → hasattr(foo, 'read'), or,
       file = _io._IOBase
 * CLOSES issue #13.
   https://github.com/isislovecruft/python-gnupg/pull/13

gnupg/gnupg.py

@@ -186,7 +186,7 @@ class GPG(GPGBase):
         else:
             log.warn("No 'default_key' given! Using first key on secring.")
 
-        if isinstance(data, file):
+        if hasattr(data, 'read'):
             result = self._sign_file(data, **kwargs)
         elif not _is_stream(data):
             stream = _make_binary_stream(data, self._encoding)