Update PyPy version in documentation

[Laerte]

The first one i bumped to match the same version of pypy3.7, after this PR was merged:

• Simplify the PyPy installation in CI #5667

I was in doubt about what version to use in second statement but both pypy3.7 and pypy3.9 use the same version, maybe we could remove this paragraph altogether since now is redundant? Version 5.9.0 is from 2017-10-05.

[codecov]

Codecov Report

Merging #5678 (c99e416) into master (e4f6545) will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5678      +/-   ##
==========================================
+ Coverage   88.69%   88.83%   +0.14%     
==========================================
  Files         162      162              
  Lines       10967    10967              
  Branches     1797     1797              
==========================================
+ Hits         9727     9743      +16     
+ Misses        960      943      -17     
- Partials      280      281       +1     

Impacted Files	Coverage Δ
scrapy/core/downloader/__init__.py	90.97% <0.00%> (-1.51%)	⬇️
scrapy/robotstxt.py	97.53% <0.00%> (+22.22%)	⬆️

[wRAR]

I don't really know what should we do with these versions, we don't explicitly set them in the CI, so the versions in the docs can easily become outdated again.

[Gallaecio]

Yes, now that we do not explicitly test them maybe we could simply remove the mentions of specific PyPy versions from the docs.

[Laerte]

@wRAR @Gallaecio Yeah i was in doubt about this too, but i totally agree with you guys, i push new commit to remove these mentions of specific PyPy versions.

[Gallaecio]

Thanks!