Update minimum python version in docs

[Laerte]

Some leftover from:

• https://github.com/scrapy/scrapyd/pull/459

CI is failing but is not because of my change.

[codecov[bot]]

Codecov Report

Merging #461 (a314e04) into master (af850e5) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #461   +/-   ##
=======================================
  Coverage   81.87%   81.87%           
=======================================
  Files          22       22           
  Lines        1004     1004           
=======================================
  Hits          822      822           
  Misses        182      182           

Flag	Coverage Δ
unittests	81.87% <ø> (ø)

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