Use session scope in qgis_bot fixture

[Joonalai]

This PR:

• Turns qgis_bot into session scoped fixture since its only requirement is session scoped as well and it has no reason to have any other scope
• Deprecates module_qgis_bot since it has no purpose anymore

[codecov-commenter]

Codecov Report

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

Comparison is base (631d92b) 56.35% compared to head (b856341) 56.44%.

Files	Patch %	Lines
src/pytest_qgis/pytest_qgis.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   56.35%   56.44%   +0.08%     
==========================================
  Files           7        7              
  Lines         488      489       +1     
==========================================
+ Hits          275      276       +1     
  Misses        213      213              

Flag	Coverage Δ
pytest	56.44% <50.00%> (+0.08%)	⬆️

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.

[LKajan]

Could you please add reasoning why this change is done to the commit body and to the PR description.