Log warning if secrets file is readable by other users

[johannaengland]

Scope and purpose

Fixes #280. I did not add an test that it actually logs the warning, but I manually tested it and added tests for the utils function.

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is. More information about contributing to Zino can be found in the README.

• [X] Added a changelog fragment for towncrier
• [X] Added/amended tests for new/changed code
• [X] Added/changed documentation
• [X] Linted/formatted the code with black, ruff and isort, easiest by using pre-commit
• [X] The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• [ ] If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.57%. Comparing base (168286a) to head (1ba3a5b). Report is 1 commits behind head on master.

Files	Patch %	Lines
src/zino/zino.py	62.50%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #342 +/- ## ========================================== - Coverage 98.60% 98.57% -0.03% ========================================== Files 77 77 Lines 9688 9701 +13 ========================================== + Hits 9552 9562 +10 - Misses 136 139 +3 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud