srpm_import: be compatible with rhbz#1693751
Older RPMs returned 'bytes' objects, newer return 'str'.
If 'str' object is returned on Python 3 - it doesn't need to be
decoded, and it actually raises error :
>>> str('', encoding='utf-8')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: decoding str is not supported
This change also OK for Python 2 where str() has decode method
(str is alternative for bytes).
Signed-off-by: Pavel Raiskup <email@example.com>