Fix intersphinx_mapping format error

[phprus]

Description

Sphinx remove support legacy intersphinx_mapping format: sphinx-doc/sphinx#12083

Documentation build error:

ERROR: Invalid value `None` in intersphinx_mapping['https://docs.python.org/']. Expected a two-element tuple or list.

Configuration error:
Invalid `intersphinx_mapping` configuration (1 error).

Fixes # - issue number(s) if exists

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• bug fix - change that fixes an issue
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• documentation - documentation update

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[phprus]

Fixed #1480