change pytest fixture to httpserver

[beliaev-maksim]

switch pytest fixture because httpserver is a way more powerful compared to localserver and has a better documentation.

also, see pytest-dev/pytest-localserver#43

[codecov]

Codecov Report

Merging #566 (f17ce37) into master (52460b9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #566   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         2636      2644    +8     
=========================================
+ Hits          2636      2644    +8     

Impacted Files	Coverage Δ
responses/tests/test_responses.py	100.00% <100.00%> (ø)

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