refactor: Cleaned codebase and improve code quality

[frgfm]

This PR introduces the following modifications:

• adds isort, and verifications for requirements and headers
• switched test suite of the client from unittest to pytest
• updated headers
• reordered imports
• removed unused imports

Any feedback is welcome!

[codecov[bot]]

Codecov Report

Merging #189 (e0d8837) into master (9c4dccb) will increase coverage by 0.74%. The diff coverage is 97.80%.

:exclamation: Current head e0d8837 differs from pull request most recent head 74c2235. Consider uploading reports for the commit 74c2235 to get more accurate results

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
+ Coverage   91.21%   91.96%   +0.74%     
==========================================
  Files          34       34              
  Lines        1229     1219      -10     
==========================================
  Hits         1121     1121              
+ Misses        108       98      -10     

Flag	Coverage Δ
unittests	91.96% <97.80%> (+0.74%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/app/api/crud/webhooks.py	100.00% <ø> (ø)
src/app/services/services.py	100.00% <ø> (ø)
src/app/services/utils.py	100.00% <ø> (ø)
src/app/api/routes/webhooks.py	77.27% <66.66%> (ø)
src/app/main.py	76.00% <80.00%> (+13.28%)	:arrow_up:
src/app/api/crud/accesses.py	97.56% <100.00%> (ø)
src/app/api/crud/alerts.py	67.74% <100.00%> (ø)
src/app/api/crud/authorizations.py	100.00% <100.00%> (ø)
src/app/api/crud/base.py	98.14% <100.00%> (ø)
src/app/api/crud/groups.py	97.91% <100.00%> (ø)
... and 20 more

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