Update dependabot settings for python packages

[matmair]

Several changes to dependabot behaviour:

• Splits backend and helper dependency updates
• schedules them for Friday
• changes version bumping schema
• adds me as an assignee as they need to be fixed by hand

[netlify]

✅ Deploy Preview for inventree-web-pui-preview canceled.

Name	Link
🔨 Latest commit	73c312c
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/676d3002dc6e9d00080c5748

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.30%. Comparing base (d4ee8c5) to head (73c312c).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8772      +/-   ##
==========================================
+ Coverage   85.27%   85.30%   +0.02%     
==========================================
  Files        1172     1172              
  Lines       51541    51541              
  Branches     2090     2090              
==========================================
+ Hits        43952    43966      +14     
- Misses       7066     7072       +6     
+ Partials      523      503      -20     

Flag	Coverage Δ
backend	86.94% <ø> (-0.04%)	⬇️
migrations	43.06% <ø> (+0.03%)	⬆️
pui	69.45% <ø> (+0.16%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.