Use Python 3.10 instead of 3.7 in installation instructions

[JPBergsma]

As we will soon drop support for python 3.7. It does not make sense to still use python 3.7 in the installation instructions.

[codecov]

Codecov Report

Merging #1229 (40935e4) into master (064b2f7) will increase coverage by 0.14%.
The diff coverage is n/a.

❗ Current head 40935e4 differs from pull request most recent head 85c2258. Consider uploading reports for the commit 85c2258 to get more accurate results

@@            Coverage Diff             @@
##           master    #1229      +/-   ##
==========================================
+ Coverage   91.27%   91.41%   +0.14%     
==========================================
  Files          72       72              
  Lines        4285     4285              
==========================================
+ Hits         3911     3917       +6     
+ Misses        374      368       -6     

Flag	Coverage Δ
project	91.41% <ø> (+0.14%)	⬆️
validator	90.66% <ø> (ø)

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Impacted Files	Coverage Δ
optimade/client/client.py	81.01% <0.00%> (+2.53%)	⬆️

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[ml-evs]

Thanks for spotting this @JPBergsma! In #1176 I use Python 3.10 as the new "main" CI version. Perhaps we should just jump straight to that?

[ml-evs]

👍