Fix docs about venv and test dependencies setup

[mfriedenhagen]

• python3 directly does include the venv module, so no need to firstly install virtualenv in your user or even site packages anymore.
• With quotes around .[test] at least the zsh of macOS does try to evaluate this itself.

[codecov]

Codecov Report

Merging #515 (43714e1) into main (7dab06c) will decrease coverage by 3.14%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
- Coverage   84.21%   81.07%   -3.14%     
==========================================
  Files          70       70              
  Lines        2679     2679              
==========================================
- Hits         2256     2172      -84     
- Misses        423      507      +84     

Impacted Files	Coverage Δ
...labform/gitlab/project_merge_requests_approvals.py	20.83% <0.00%> (-66.67%)	⬇️
...itlabform/gitlab/project_protected_environments.py	42.85% <0.00%> (-35.72%)	⬇️
...orm/processors/project/merge_requests_approvals.py	70.00% <0.00%> (-30.00%)	⬇️
gitlabform/processors/util/branch_protector.py	60.68% <0.00%> (-13.68%)	⬇️
gitlabform/processors/single_entity_processor.py	81.81% <0.00%> (-9.10%)	⬇️
gitlabform/gitlab/projects.py	60.82% <0.00%> (-8.25%)	⬇️
gitlabform/gitlab/branches.py	80.70% <0.00%> (-7.02%)	⬇️
gitlabform/gitlab/core.py	90.37% <0.00%> (-4.45%)	⬇️
gitlabform/processors/abstract_processor.py	75.82% <0.00%> (-3.30%)	⬇️
gitlabform/processors/project/members_processor.py	86.15% <0.00%> (-3.08%)	⬇️
... and 1 more

[gdubicki]

Thanks for your contribution @mfriedenhagen! 😊