Stop resource leakage when reading conf files

[lunkwill42]

This should fix the annoying ResourceWarnings coming through during the test suite:

/source/python/nav/config.py:339: ResourceWarning: unclosed file <_io.TextIOWrapper name='/source/.tox/integration-py37-django32/etc/nav.conf' mode='r' encoding='utf-8'>
  NAV_CONFIG = read_flat_config('nav.conf')
ResourceWarning: Enable tracemalloc to get the object allocation traceback
/source/python/nav/django/settings.py:36: ResourceWarning: unclosed file <_io.TextIOWrapper name='/source/.tox/integration-py37-django32/etc/webfront/webfront.conf' mode='r' encoding='utf-8'>
  _webfront_config = getconfig('webfront/webfront.conf')
ResourceWarning: Enable tracemalloc to get the object allocation traceback
/source/python/nav/db/__init__.py:104: ResourceWarning: unclosed file <_io.TextIOWrapper name='/source/.tox/integration-py37-django32/etc/db.conf' mode='r' encoding='utf-8'>
  conf = config.read_flat_config('db.conf')
ResourceWarning: Enable tracemalloc to get the object allocation traceback

This PR just ensures we wrap the config-reading code in a context manager so the config files always end up being closed.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
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No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #2451 (743de9d) into master (26c69f1) will increase coverage by 0.00%.
The diff coverage is 90.47%.

@@           Coverage Diff           @@
##           master    #2451   +/-   ##
=======================================
  Coverage   52.72%   52.73%           
=======================================
  Files         552      552           
  Lines       40186    40188    +2     
=======================================
+ Hits        21190    21192    +2     
  Misses      18996    18996           

Impacted Files	Coverage Δ
python/nav/config.py	64.96% <90.47%> (+0.45%)	⬆️

[github-actions]

Test results

     12 files       12 suites   12m 3s ⏱️
3 088 tests 2 992 ✔️   96 💤 0 ❌
8 739 runs  8 451 ✔️ 288 💤 0 ❌

Results for commit 743de9d.

[johannaengland]

For the functional tests this error message still appears:

/source/tests/functional/conftest.py:30: ResourceWarning: unclosed file <_io.BufferedWriter name='reports/gunicorn.log'>
  start_gunicorn()
ResourceWarning: Enable tracemalloc to get the object allocation traceback

[lunkwill42]

For the functional tests this error message still appears:

/source/tests/functional/conftest.py:30: ResourceWarning: unclosed file <_io.BufferedWriter name='reports/gunicorn.log'>
  start_gunicorn()
ResourceWarning: Enable tracemalloc to get the object allocation traceback

Possibly, but this is not a config file leakage, it's part of the test suite setup, and does not affect production code (i.e. separate PR, at best)

[johannaengland]

Works as intended 👍