[docs] [issue35630] Missing code tag for "python3" in README.rst

[Suriyaa Sundararuban]

Change by Suriyaa Sundararuban <python at suriyaa.tk>:


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keywords: +patch, patch, patch
pull_requests: +10767, 10768, 10769
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35630>
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