Add keyword to get response from last request

[PaulBrandUWV]

This Pull request adds a keyword to get the response from the last request.
The keyword can be used to create much cleaner testcases because the response does not have to be stored to a variable in the test case.

Example:

*** Settings ***
Library     RequestsLibrary

*** Variables ***
${url}    https://httpbin.org/base64
${data}    anlmhcrh7853r042yqtx7rhaohfx,jr0aofds*&*^$IHLFEMXhkdjsxhcc
${base64_data}    YW5sbWhjcmg3ODUzcjA0MnlxdHg3cmhhb2hmeCxqcjBhb2ZkcyomKl4kSUhMRkVNWGhrZGpzeGhjYw==

*** Keywords ***
Response body equals
    [Arguments]    ${data}
    ${response}=   Get response
    Should be equal    ${response.text}    ${data}

*** Test cases ***
httpbin base64 decode
    When get  ${url}/${base64_data}
    Then response body equals    ${data}

This PR also removes some skipif decorators from the unit tests for old python versions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.67%. Comparing base (8a42d5c) to head (ea3cd17).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   84.94%   88.67%   +3.72%     
==========================================
  Files           9        9              
  Lines         465      468       +3     
  Branches      116      117       +1     
==========================================
+ Hits          395      415      +20     
+ Misses         65       51      -14     
+ Partials        5        2       -3     

Flag	Coverage Δ
acceptance	85.04% <100.00%> (+0.09%)	⬆️
unit	59.61% <66.66%> (?)

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[PaulBrandUWV]

@lucagiove what do you think?