Switch from requests to httpx

[jodal]

• Switch from requests to httpx
• Reuse HTTP connections across requests

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (4328ed8) 88.13% compared to head (d010da9) 85.92%.

Files	Patch %	Lines
src/brreg/enhetsregisteret/_client.py	75.00%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   88.13%   85.92%   -2.21%     
==========================================
  Files           5        5              
  Lines         118      135      +17     
  Branches       19       20       +1     
==========================================
+ Hits          104      116      +12     
- Misses         10       14       +4     
- Partials        4        5       +1     

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