Add conservative_retrying

[Gallaecio]

I feel like this alternative retry policy will be more useful to most projects. Since changing the default retry policy to something like this would require some discussion, I think it may be a good idea to implement it as an alternative ready-to-use retry policy, so that fewer users need to implement their own custom retry policy.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9e5bb74) to head (d87c0e6).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #71   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines          469       558   +89     
  Branches        56        71   +15     
=========================================
+ Hits           469       558   +89     

Files	Coverage Δ
zyte_api/__init__.py	100.00% <100.00%> (ø)
zyte_api/_async.py	100.00% <100.00%> (ø)
zyte_api/_retry.py	100.00% <100.00%> (ø)

[Gallaecio]

Closing and re-opening to verify a fix for the unrelated issue of Codecov uploads…

[kmike]

Thanks @Gallaecio!