Lock down Cython language level.
FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: /Users/mikeboers/Documents/PyAV/av/filter/context.pxdFutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: /Users/mikeboers/Documents/PyAV/av/filter/context.pxd