[C][Python] Vendor pythoncapi-compat instead of adjusting changes manually

[raulcd]

Describe the enhancement requested

This issue is an enhancement follow up from this comment: #42034 (comment)

Based on the discussion:
Vendoring it should be trivial: there is a single .h file.

Component(s)

C++, Python

[jorisvandenbossche]

Issue resolved by pull request 43767
#43767