Simplify PyPy installation, bump actions versions

[Laerte]

Based on this PR done in Scrapy:

• https://github.com/scrapy/scrapy/pull/5667

Kudos to @wRAR 🎉

[codecov[bot]]

Codecov Report

Merging #30 (be58a17) into master (242c2fe) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files           1        1           
  Lines         307      307           
  Branches       74       71    -3     
=======================================
  Hits          300      300           
  Misses          4        4           
  Partials        3        3           

[Gallaecio]

Thanks!