Ensure cache directory exists when attempting to use it

[pR0Ps]

Even though the cache directory is created when the object is initialized, it's possible for it to be deleted while the program is running, leading to failures to create the temporary files.

In my specific case, I have the cache set up to use a subdirectory of /tmp, which is occasionally cleaned up by the operating system. This leads to errors like the following when attempting to cache transcodes or extract album art:

Traceback (most recent call last):
  File "/usr/lib/python3.12/site-packages/werkzeug/wsgi.py", line 256, in __next__
    return self._next()
  File "/usr/lib/python3.12/site-packages/werkzeug/wrappers/response.py", line 32, in _iter_encoded
    for item in iterable:
  File "/usr/lib/python3.12/site-packages/supysonic/cache.py", line 183, in set_generated
    with self.set_fileobj(key) as f:
  File "/usr/lib/python3.12/contextlib.py", line 137, in __enter__
    return next(self.gen)
  File "/usr/lib/python3.12/site-packages/supysonic/cache.py", line 144, in set_fileobj
    f = tempfile.NamedTemporaryFile(
  File "/usr/lib/python3.12/tempfile.py", line 582, in NamedTemporaryFile
    file = _io.open(dir, mode, buffering=buffering,
  File "/usr/lib/python3.12/tempfile.py", line 579, in opener
    fd, name = _mkstemp_inner(dir, prefix, suffix, flags, output_type)
  File "/usr/lib/python3.12/tempfile.py", line 256, in _mkstemp_inner
    fd = _os.open(file, flags, 0o600)
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/supysonic/transcodes/tmp8dnfpyvr.part'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.56%. Comparing base (1f3471c) to head (ea92180).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #274   +/-   ##
=======================================
  Coverage   95.56%   95.56%           
=======================================
  Files          54       54           
  Lines        4310     4311    +1     
=======================================
+ Hits         4119     4120    +1     
  Misses        191      191           

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[spl0k]

Hello!

Since a transcode can take some time, isn't there a window where a cleanup can happen between the temporary file creation and its rename on line 154? That would most likely raise the same kind of error there too.