[ntfy] Fix submitting newline characters in notification message text

[amotl]

Dear @codebude and @zoic21,

this patch has been grown in July already, but only now I have been able to wrap it up. It is based on the suggestion by @codebude shared at https://github.com/mqtt-tools/mqttwarn/issues/677#issuecomment-1575060446, thank you very much.

With kind regards, Andreas.

P.S.: I will be happy about a review, both on code and/or functionality, either before, or after merging and releasing. If it's about code, commenting on this PR will be the appropriate spot. If it is about the outcome, I'd say we just continue our conversation at GH-677, independently of this PR.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6a24a8a) 49.33% compared to head (731afef) 49.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #684 +/- ## ========================================== + Coverage 49.33% 49.51% +0.17% ========================================== Files 81 81 Lines 3973 3993 +20 ========================================== + Hits 1960 1977 +17 - Misses 2013 2016 +3 ``` | [Flag](https://app.codecov.io/gh/mqtt-tools/mqttwarn/pull/684/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mqtt-tools) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/mqtt-tools/mqttwarn/pull/684/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mqtt-tools) | `49.51% <96.42%> (+0.17%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mqtt-tools#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/mqtt-tools/mqttwarn/pull/684?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mqtt-tools) | Coverage Δ | | |---|---|---| | [mqttwarn/services/ntfy.py](https://app.codecov.io/gh/mqtt-tools/mqttwarn/pull/684?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mqtt-tools#diff-bXF0dHdhcm4vc2VydmljZXMvbnRmeS5weQ==) | `97.27% <96.42%> (-1.94%)` | :arrow_down: |

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[codebude]

Great job! I was looking forward to this release. Thanks! :-)

[amotl]

Wonderful. If this works now, we can close GH-677?

/cc @zoic21