Explicitly close encoded file

[NicolasHug]

Fixes #724

I hope that this is the fix for the errors we're seeing in #724.:

FAILED test/test_encoders.py::TestAudioEncoder::test_num_channels[to_file-2-2] - RuntimeError: Could not open input file: /tmp/pytest-of-root/pytest-0/test_num_channels_to_file_2_2_0/output.mp3 Invalid data found when processing input

I think the error was that we were trying to read an encoded file that we had just encoded, and because that file wasn't fully close()ed yet, something was going wrong.

Hard to test for this. Let's monitor the repo and see if we still notice the same kind of issues.