python3: introduce ABI_VERSION flag

Related to discussion:
  #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 especially if they are linking against the shared
binary libraries.

To do this, we add an ABI_VERSION option in Python3's Makefile.

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

lang/python/python3/Makefile

@@ -11,7 +11,7 @@ include $(TOPDIR)/rules.mk
 include ../python3-version.mk
 
 PKG_NAME:=python3
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 PKG_VERSION:=$(PYTHON3_VERSION).$(PYTHON3_VERSION_MICRO)
 
 PKG_SOURCE:=Python-$(PKG_VERSION).tar.xz
@@ -23,6 +23,8 @@ PKG_LICENSE:=Python/2.0
 PKG_LICENSE_FILES:=LICENSE Doc/copyright.rst Doc/license.rst Modules/_ctypes/darwin/LICENSE Modules/_ctypes/libffi_osx/LICENSE Modules/expat/COPYING
 PKG_CPE_ID:=cpe:/a:python:python
 
+ABI_VERSION:=$(PYTHON3_VERSION_MAJOR).$(PYTHON3_VERSION_MINOR)
+
 # This file provides the necsessary host build variables
 include ../python3-host.mk