Added line decoding before sniffing at PythonParser#_make_reader
Fixes issue when reading CSV from unencoded IO stream (i.e. WSGI request body). CSV sniffer uses regexp internally and crashes with `TypeError: cannot use a string pattern on a bytes-like object`.
This change fixes the issue. Better way, however would be to add proper encoding support to `csv.Sniffer()`.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
