werkzeug deprecation warnings fix

[csernazs]

• poetry: update python to >=3.8 to install the latest werkzeug
• httpserver.py: remove the now deprecated werkzeug.urls.url_decode call
• httpserver.py: fix werkzeug deprecated parse_authorization_header call

[csernazs]

Fixes #242.

[codecov[bot]]

Codecov Report

Merging #243 (c1139d7) into master (1df798a) will decrease coverage by 0.14%. The diff coverage is 87.50%.

:exclamation: Current head c1139d7 differs from pull request most recent head a803cf6. Consider uploading reports for the commit a803cf6 to get more accurate results

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
- Coverage   95.61%   95.47%   -0.14%     
==========================================
  Files           4        4              
  Lines         593      597       +4     
==========================================
+ Hits          567      570       +3     
- Misses         26       27       +1     

Impacted Files	Coverage Δ
pytest_httpserver/httpserver.py	95.57% <87.50%> (-0.19%)	:arrow_down: