imgtool: fixed keys/general.py to pass existing unittests

keys.KeyClass._emit is able to use 'file' parameter not as a file
but some object (not only sys.stdout but io.StringIO, like by
tests).

Signed-off-by: Denis Mingulov <denis@mingulov.com>

scripts/imgtool/keys/general.py

@@ -2,6 +2,7 @@
 
 # SPDX-License-Identifier: Apache-2.0
 
+import io
 import sys
 from cryptography.hazmat.primitives.hashes import Hash, SHA256
 
@@ -11,13 +12,13 @@
 class KeyClass(object):
     def _emit(self, header, trailer, encoded_bytes, indent, file=sys.stdout,
               len_format=None):
-        if file and file is not sys.stdout:
+        if file and not isinstance(file, io.TextIOBase):
             with open(file, 'w') as file:
                 self._emit_to_output(header, trailer, encoded_bytes, indent,
                                      file, len_format)
         else:
             self._emit_to_output(header, trailer, encoded_bytes, indent,
-                                 sys.stdout, len_format)
+                                 file, len_format)
 
     def _emit_to_output(self, header, trailer, encoded_bytes, indent, file,
                         len_format):