pylint fixes

--HG--
branch : pure-cffi
avformat_find_stream can fail without consequences.

at elast as first try, we treat those failure as not serious,
so we don't detect nor traslate those errors into exceptions.

--HG--
branch : pure-cffi
add tests and restore the coverage.

--HG--
branch : pure-cffi
add an extra newline to be compliant with pep8.

--HG--
branch : pure-cffi
pep8 fixes

--HG--
branch : pure-cffi
Merge ../pyrana