TypeError if charset is in handle of dot file

[n-piipel]

Error appears If charset is in .dot file.
The call of the method new on line 1616 waits to get graph.encoding of string, but it gets bytes, because:
C-function agget (from line 181 from agraph.py) returns bytes, but it must be string.
And C-func agattrdefval (from line 1838) returns bytes too.
And I think the same return is from C-func agattrdefval from line 1746.