Fix ascii_filename to work properly in Python 3

[kwikwag]

This fixes issue #49. In Python 3 the result of str.encode() is bytes and so "something %s" % (b'else') == "something b'else'", which 2to3does not catch because of thestr-bytes` ambiguity in Python 2 (see also here). I suggest simply reconverting the ASCII-bytes into a Unicode string (containing only ASCII characters) for Python 3. Actually, the version check is redundant, as in Python 2 I think

s = s.encode('ascii', 'ignore')
assert s.decode('ascii') == s

Is always true.

I haven't tested this code.