Mk/Uses/python.mk: introduce PYTHON_EXT_SUFFIX variable

Since Python-3.8.7 it uses unified with other platforns shared libs naming scheme:
instead of plain .so suffix now it refers to used cpython: .cpython-${PYTHON_SUFFIX}.so

Future Python releases will use this scheme too, it will be hooked via version check

PR:		252057
Approved by:	python (wen)

Mk/Uses/python.mk

@@ -442,6 +442,13 @@ PYTHON_MAJOR_VER=	${PYTHON_VER:R}
 PYTHON_REL=		# empty
 PYTHON_ABIVER=		# empty
 PYTHON_PORTSDIR=	${_PYTHON_RELPORTDIR}${PYTHON_SUFFIX}
+
+.if ${PYTHON_VER} == 3.8
+PYTHON_EXT_SUFFIX=	.cpython-${PYTHON_SUFFIX}
+.else
+PYTHON_EXT_SUFFIX=	# empty
+.endif
+
 # Protect partial checkouts from Mk/Scripts/functions.sh:export_ports_env().
 .if !defined(_PORTS_ENV_CHECK) || exists(${PORTSDIR}/${PYTHON_PORTSDIR})
 .include "${PORTSDIR}/${PYTHON_PORTSDIR}/Makefile.version"
@@ -698,6 +705,7 @@ PLIST_SUB+=	PYTHON_INCLUDEDIR=${PYTHONPREFIX_INCLUDEDIR:S;${PREFIX}/;;} \
 		PYTHON_PLATFORM=${PYTHON_PLATFORM} \
 		PYTHON_SITELIBDIR=${PYTHONPREFIX_SITELIBDIR:S;${PREFIX}/;;} \
 		PYTHON_SUFFIX=${PYTHON_SUFFIX} \
+		PYTHON_EXT_SUFFIX=${PYTHON_EXT_SUFFIX} \
 		PYTHON_VER=${PYTHON_VER} \
 		PYTHON_VERSION=${PYTHON_VERSION}
 .if ${PYTHON_REL} < 3000