Lint: Fix type errors

[ByteOtter]

What this PR does / why we need it:

Fixes the remaining typing errors thrown by mypy.

Adds two new dependencies: boto3-stubs for boto3 type information and mypy as dev dependency to allow setup of the dev environment via poetry.

Which issue(s) this PR fixes:
Fixes #152

Special notes for your reviewer:

Release note:

[codecov]

Codecov Report

❌ Patch coverage is 96.62921% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.55%. Comparing base (9bc3c1f) to head (afc8be6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/distro_version/__init__.py	95.45%	1 Missing ⚠️
src/gardenlinux/git/repository.py	66.66%	1 Missing ⚠️
src/gardenlinux/github/release/__init__.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #296   +/-   ##
=======================================
  Coverage   91.54%   91.55%           
=======================================
  Files          42       42           
  Lines        2070     2083   +13     
=======================================
+ Hits         1895     1907   +12     
- Misses        175      176    +1     

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[NotTheEvilOne]

LGTM so far. We still need to work on #189 to better integrate this code.

[NotTheEvilOne]

Not sure if I find this syntax better readable even thought I understand the conditional import...

[ByteOtter]

Thanks for your feedback. I think I know which section you are refering to ... but I think it's unavoidable here sadly.