Refactor transports

[leszekhanusz]

• split requests transport in private methods similarly than the aiohttp and httpx transport
• remove the _prepare_batch_request method and use _prepare_request instead for batch requests to avoid code duplication

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fb869a) to head (eaf6d1b).
Report is 20 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #557    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3092   +184     
==========================================
+ Hits          2908      3092   +184     

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