python3: enforce version assignment

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

lang/python3/files/python3-package.mk

@@ -5,8 +5,8 @@
 # See /LICENSE for more information.
 #
 
-PYTHON_VERSION=3.4
-PYTHON_VERSION_MICRO=1
+PYTHON_VERSION:=3.4
+PYTHON_VERSION_MICRO:=1
 
 PYTHON_DIR:=$(STAGING_DIR)/usr
 PYTHON_BIN_DIR:=$(PYTHON_DIR)/bin