Drop CPython 3.6 support

[itzpr3d4t0r]

Python 3.6 is officially dead:

I propose we drop compatibility for 3.6. This would allow us to lower CI runtime and let us use stuff that 3.7+ only supports, such as FASTCALL (speaking of C API).

[MightyJosip]

Looking at some of the most used python libraries on PyPi (or documentation page if PyPi doesn't have Requires section)

library	version supported
pandas	3.8
numpy	3.8
keras	3.7
tensorflow	3.7
scipy	3.8
pytorch	3.7
django	3.8
flask	3.7
opencv	3.6

[pmp-p]

i think opencv is abi3 3.7+, at least on already built available wheel distribution for all OSes
( according to https://pypi.org/project/opencv-python/#files )
sidenote : 3.6 serie has never worked correctly on any android version. 3.7 is minimal version to support it.