utf-8 fix in unpickle

[piotr1212]

should fix #2652

[obfuscurity]

Per @adamcstephens' fix, we should probably be passing utf8 instead of bytes. Looks like the builds are broken, but otherwise this change appears to be working for us in limited testing.

[piotr1212]

@obfuscurity Thanks for reporting. We are probably breaking Python 2 support with this change, thus the failed tests.

[piotr1212]

hmm, well actually py2 should still work, only the test is incompatible.

[ploxiln]

I have a possibly better example that may work as intended for both python2.7 and python3: b"S'test.d\\xc3\\xb8d'\np0\n."

python2.7:

>>> s = 'test.død'
>>> pickle.dumps(s)
"S'test.d\\xc3\\xb8d'\np0\n."
>>> pickle.loads(b"S'test.d\\xc3\\xb8d'\np0\n.")
'test.d\xc3\xb8d'
>>> print(pickle.loads(b"S'test.d\\xc3\\xb8d'\np0\n."))
test.død

python3.7:

>>> pickle.loads(b"S'test.d\\xc3\\xb8d'\np0\n.")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 6: ordinal not in range(128)
>>> pickle.loads(b"S'test.d\\xc3\\xb8d'\np0\n.", encoding='utf8')
'test.død'

[deniszh]

I added test in #2660, let's continue there.