python3: introduce libpython3 with ABI_VERSION flag

python3: introduce libpython3 with ABI_VERSION flag

Related to discussion:
  https://github.com/openwrt/packages/pull/14060

Every once in a while a version bump will occur that requires an ABI
change. Example: Python 3.8 to 3.9. When this happens some Python packages
would need to be rebuilt. In setups where everything gets rebuilt, this
isn't a problem.

It's usually a bigger problem when needing to upgrade something via
opkg.
To accommodate for this, we add a libpython with it's own ABI_VERSION
flag. If this ABI_VERSION changes, then this should propagate forward.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>