Use default interval from pollfile to stagger new jobs

[johannaengland]

Scope and purpose

Fixes #337.

Changed the way that read_polldevs work to make it easier to return the PollDevices and the default values. I was considering splitting the function in two: One to read the defaults and one to read the rest - the poll devices, but decided for this method, since it is simpler changes.

Contributor Checklist

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• [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 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.20%. Comparing base (70b1a57) to head (a3300ec).

Files	Patch %	Lines
src/zino/getuptime.py	66.67%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #339 +/- ## ======================================= Coverage 98.20% 98.20% ======================================= Files 64 64 Lines 7926 7930 +4 ======================================= + Hits 7783 7787 +4 Misses 143 143 ```

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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