[Fuzzard]

ensure to have at least python version 3.7 or above

Not that easy with Ubuntu LTS, without breaking stuff. AFAIR, I can break system to the point it won't boot - and just installing it side by side won't work as apps won't pick up default Python installation.
Is it necessary to use that version of Python for building plugin? If it is, I'll have to reinstall whole system to have newer updates.

Yes, that is why I suggested to move to Ubuntu 20.04 LTS.
Kodi 19 relies on Python 3.7.5 or above and on Windows and Android comes with it. On linux it relies on System Python and with Ubuntu 18.04LTS stuck at Python 3.6 can't do much. SO if you want to use Ubuntu 18.04LTS, then stick with Kodi 18.9 and if you want to move to Kodi19.x move to Ubuntu 20.04LTS first

Just to clarify, kodi itself doesnt rely on python 3.7 as minimum. It will work with 3.6. However if addon devs utilise features/modules that arent available in 3.6 (which this sounds like the case) then you are going to come across these issues.

Any platform that isnt linux currently uses 3.8.5 as a static library, but linux uses system shared libs. We have kept 3.6+ compatiblity with the python capi because of the linux usecase. Addon devs can do as they wish around module usage.

The advise by castagnaIT regarding the backport module of contextvars to 3.6 may be the best work around for this user.