stream not properly closed (NoBackendError | OSError: Too many open files)

```
import audioread

filename = 'some_audio_file.ogg'
try:
for K in range(2048):
with audioread.audio_open(filename) as audio:
print(K, audio.duration)
except audioread.exceptions.NoBackendError:
with open(filename, 'rb') as file: # OSError: [Errno 24] Too many open files
pass
```
the audio is openned in a loop using with, but it seems that is not properly closed
in my system it will print about 500 lines, then raises the NoBackendError and the OSError when trying to open a new file

using `lsof` shows lines of the form (my limit is 1024)
```
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
python 38124 ne555 1023u unix 0x00000000d813f451 0t0 2420780 type=STREAM (CONNECTED)
```

I've observed this issue with audioread.gstdec.GstAudioFile and audioread.ffdec.FFmpegAudioFile backends, couldn't test with audioread.rawread.RawAudioFile

Start with the repeated audioread.audio_open() example and inspect the GstAudioFile and FFmpegAudioFile backends named in the report, comparing their resource cleanup with RawAudioFile. Reproduce the loop while monitoring open descriptors with lsof; done means repeated opens and closes do not exhaust file descriptors and the reported exceptions no longer occur.

Written by the indexing model from the issue text.