Dependencies: Update `requests-toolbelt` for compatibility w. urllib3 2.x

[amotl]

What the title says. I think the update to urllib3 2.x, which has been released just recently, is responsible for this error. It has been revealed at the CI run of GH-649.

/opt/hostedtoolcache/Python/3.8.16/x64/lib/python3.8/site-packages/_pytest/assertion/rewrite.py:172: in exec_module
    exec(co, module.__dict__)
tests/services/test_pushover.py:8: in <module>
    from requests_toolbelt import MultipartDecoder
/opt/hostedtoolcache/Python/3.8.16/x64/lib/python3.8/site-packages/requests_toolbelt/__init__.py:12: in <module>
    from .adapters import SSLAdapter, SourceAddressAdapter
/opt/hostedtoolcache/Python/3.8.16/x64/lib/python3.8/site-packages/requests_toolbelt/adapters/__init__.py:12: in <module>
    from .ssl import SSLAdapter
/opt/hostedtoolcache/Python/3.8.16/x64/lib/python3.8/site-packages/requests_toolbelt/adapters/ssl.py:16: in <module>
    from .._compat import poolmanager
/opt/hostedtoolcache/Python/3.8.16/x64/lib/python3.8/site-packages/requests_toolbelt/_compat.py:50: in <module>
    from urllib3.contrib import appengine as gaecontrib
E   ImportError: cannot import name 'appengine' from 'urllib3.contrib' (/opt/hostedtoolcache/Python/3.8.16/x64/lib/python3.8/site-packages/urllib3/contrib/__init__.py)

-- https://github.com/jpmens/mqttwarn/actions/runs/4896318706/jobs/8742985944?pr=649#step:7:60

[codecov-commenter]

Codecov Report

Merging #655 (20b1131) into main (3baaf2b) will increase coverage by 0.15%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
+ Coverage   46.00%   46.15%   +0.15%     
==========================================
  Files          80       80              
  Lines        3900     3913      +13     
==========================================
+ Hits         1794     1806      +12     
- Misses       2106     2107       +1     

Flag	Coverage Δ
unittests	46.15% <ø> (+0.15%)	:arrow_up:

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see 11 files with indirect coverage changes

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