ci: use Python 3.5.3 in AppVeyor
ci: use Python 3.5.3 in AppVeyor

3.5.2 is broken for some reason. Let's try 3.5.3 to see if that fixes
it.
terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard