Drop Python3 build requirement

This is only being used for extracting the package name from package
json, to avoid having to duplicate it in `Makefile`.
But for only this purpose, Python 3 is rather heavyweight. It's also not
available on RHEL/CentOS 7.

Makefile

@@ -1,4 +1,5 @@
-PACKAGE_NAME := $(shell python3 -c "import json; print(json.load(open('package.json'))['name'])")
+# extract name from package.json
+PACKAGE_NAME := $(shell awk '/"name":/ {gsub(/[",]/, "", $$2); print $$2}' package.json)
 VERSION := $(shell git describe 2>/dev/null || echo 1)
 ifeq ($(TEST_OS),)
 TEST_OS = centos-7