config file not properly closed

Hi,
I maybe mis-suing the `confuse` library but I am running a function which uses a method like:

```python
import confuse
import logging

def get_logpath():
try:
config = confuse.Configuration("mmdtool", __name__)
logfilepath = config["paths"]["logs"].get()
except NotFoundError:
logfilepath = "./logs/"
if not pathlib.Path(logfilepath).exists():
pathlib.Path(logfilepath).mkdir(parents=True, exist_ok=True)
return logfilepath
```
it works fine for a while but as the number of files processed increase, when I run my script in parallel over thousands of record, at some point the parallel job breaks with the following error:

```
ubuntu@pycsw-prod:/mnt/csw/dev/py-mmd-tools/script$ python3 convert_all.py -i /mnt/csw/metadata/nbs -t /mnt/csw/dev/mmd/xslt/mmd-to-iso.xsl -o /mnt/csw/metadata/nbs_iso/
os.walk("/mnt/csw/metadata/nbs")
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/home/ubuntu/.local/lib/python3.8/site-packages/confuse/yaml_util.py", line 85, in load_yaml
OSError: [Errno 24] Too many open files: '/home/ubuntu/.config/mmdtool/config.yaml'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.8/multiprocessing/pool.py", line 125, in worker
File "/usr/lib/python3.8/multiprocessing/pool.py", line 48, in mapstar
File "/usr/local/lib/python3.8/dist-packages/parmap/parmap.py", line 104, in _func_star_single
File "convert_all.py", line 33, in writerecord
File "/mnt/csw/dev/py-mmd-tools/py_mmd_tools/mmd_to_csw_iso.py", line 40, in mmd_to_iso
File "/mnt/csw/dev/py-mmd-tools/py_mmd_tools/mmd_util.py", line 31, in setup_log
File "/mnt/csw/dev/py-mmd-tools/py_mmd_tools/mmd_util.py", line 21, in get_logpath
File "/home/ubuntu/.local/lib/python3.8/site-packages/confuse/core.py", line 558, in __init__
File "/home/ubuntu/.local/lib/python3.8/site-packages/confuse/core.py", line 600, in read
File "/home/ubuntu/.local/lib/python3.8/site-packages/confuse/core.py", line 574, in _add_user_source
File "/home/ubuntu/.local/lib/python3.8/site-packages/confuse/yaml_util.py", line 88, in load_yaml
confuse.exceptions.ConfigReadError: file /home/ubuntu/.config/mmdtool/config.yaml could not be read: [Errno 24] Too many open files: '/home/ubuntu/.config/mmdtool/config.yaml'
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "convert_all.py", line 56, in
main(metadata=args.input_dir, mmd2iso_xslt=args.input_xslt, outdir=args.output_dir)
File "convert_all.py", line 42, in main
y = parmap.map(writerecord, xmlfiles, mmd2iso_xslt=mmd2iso_xslt, outdir=outdir, pm_pbar=False)
File "/usr/local/lib/python3.8/dist-packages/parmap/parmap.py", line 304, in map
return _map_or_starmap(function, iterable, args, kwargs, "map")
File "/usr/local/lib/python3.8/dist-packages/parmap/parmap.py", line 248, in _map_or_starmap
output = result.get()
File "/usr/lib/python3.8/multiprocessing/pool.py", line 768, in get
raise self._value
confuse.exceptions.ConfigReadError: file file /home/ubuntu/.config/mmdtool/config.yaml could not be read: [Errno 24] Too many open files: '/home/ubuntu/.config/mmdtool/config.yaml' could not be read
```

I tried to replace my code with:

```python
with confuse.Configuration("mmdtool", __name__) as config:
logfilepath = config["paths"]["logs"].get()
```

with the hope to get the config file closed, but that didn't work as I got a `AttributeError: __enter__`