Fixing incompatibilities with Python 3.9 and some small style issues

[vlastocom]

Fixes:

• The problem with removed base64.encodestring() and base64.decodestring() methods
• The problem with Python3 implementation of parse_n3_term() (the method relied on protected fields of ast library objects and their semantics changed, which meant that the ValueError would be returned on parsing strings, for example)
• Some minor issues highlighted by PEP linter (long lines, indentation)